Memory device, semiconductor device, and electronic device

ABSTRACT

An object is to provide a memory device which does not need a complex manufacturing process and whose power consumption can be suppressed, and a semiconductor device including the memory device. A solution is to provide a capacitor which holds data and a switching element which controls storing and releasing charge in the capacitor in a memory element. In the memory element, a phase-inversion element such as an inverter or a clocked inverter includes the phase of an input signal is inverted and the signal is output. For the switching element, a transistor including an oxide semiconductor in a channel formation region is used. In the case where application of a power supply voltage to the phase-inversion element is stopped, the data is stored in the capacitor, so that the data is held in the capacitor even when the application of the power supply voltage to the phase-inversion element is stopped.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/744,860, filed Jan. 18, 2013, now allowed, which is continuation of U.S. application Ser. No. 12/976,206, filed Dec. 22, 2010, now U.S. Pat. No. 8,362,538, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2009-293982 on Dec. 25, 2009, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a memory device, a semiconductor device including the memory device, and an electronic device including the memory device.

BACKGROUND ART

A transistor including a semiconductor film which is formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since there is limitation on manufacture of a transistor in terms of allowable temperature limit of a substrate, a transistor including, in an active layer, amorphous silicon that can be deposited at relatively low temperatures, polysilicon that can be obtained by crystallization with use of a laser beam or a catalytic element, or the like is mainly used for a semiconductor display device.

In recent years, a metal oxide showing semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material having high mobility obtained by polysilicon or microcrystalline silicon and having uniform element characteristics obtained by amorphous silicon. The metal oxide is used for various applications; for example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides showing semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide, and transistors in each of which a channel formation region is formed using such a metal oxide showing semiconductor characteristics have been known (Patent Documents 1 and 2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2007-123861 -   [Patent Document 2] Japanese Published Patent Application No.     2007-096055

DISCLOSURE OF INVENTION

By the way, a register, which is one of memory devices, is a logic circuit generally including a logic element such as an inverter or a clocked inverter, and a switching element. Since the operation speed of the register is far higher than that of a random access memory (RAM) which is used as a main memory, the register is used in a processor in order to temporally hold data for holding an arithmetic processing, a program execution state, or the like.

In FIG. 16A, one memory element included in a register in which inverters are used is illustrated. A register 1300 illustrated in FIG. 16A includes an inverter 1301, an inverter 1302, a switching element 1303, and a switching element 1304. Input of a signal IN to an input terminal of the inverter 1301 is controlled by the switching element 1303. A potential of an output terminal of the inverter 1301 is supplied to a circuit of a subsequent stage as a signal OUT. The output terminal of the inverter 1301 is connected to an input terminal of the inverter 1302, and an output terminal of the inverter 1302 is connected to the input terminal of the inverter 1301 via the switching element 1304.

When the switching element 1303 is turned off and the switching element 1304 is turned on, a potential of the signal IN which is input via the switching element 1303 is held in the register 1300.

A more specific circuit configuration of the register 1300 of FIG. 16A is illustrated in FIG. 16B. The register 1300 illustrated in FIG. 16B includes the inverter 1301, the inverter 1302, the switching element 1303, and the switching element 1304. The connection structure of these circuit elements are the same as that in FIG. 16A.

The inverter 1301 includes a p-channel transistor 1310 and an n-channel transistor 1311 whose gate electrodes are connected to each other. In addition, the p-channel transistor 1310 and the n-channel transistor 1311 are connected in series between a node to which a high-level power supply potential VDD is supplied and a node to which a low-level power supply potential VSS is supplied. In a similar manner, the inverter 1302 includes a p-channel transistor 1312 and an n-channel transistor 1313 whose gate electrodes are connected to each other. In addition, the p-channel transistor 1312 and the n-channel transistor 1313 are connected in series between a node to which a high-level power supply potential VDD is supplied and a node to which a low-level power supply potential VSS is supplied.

The inverter 1301 illustrated in FIG. 16B operates such that one of the p-channel transistor 1310 and the n-channel transistor 1311 is turned on and the other is turned off according to the level of potentials supplied to the gate electrodes thereof. Thus, current between the node to which the power supply potential VDD is supplied and the node to which the power supply potential VSS is supplied should be ideally zero. However, actually a minute amount of OFF-state current flows in the OFF-state transistor; therefore, the current between the nodes can not be zero. A similar phenomenon occurs in the inverter 1302; as a result, power is consumed in the register 1300 even in a hold state in which data is not written.

In the case of an inverter manufactured using bulk silicon for example, an OFF-state current of about 1 pA is generated at room temperature at a voltage between nodes of about 3 V, although it depends on the size of a transistor. The memory element illustrated in FIGS. 16A and 16B includes two inverters: the inverter 1301 and the inverter 1302; therefore, an OFF-state current of about 2 pA is generated. In the case of a register including about 10⁷ memory elements, the OFF-state current of the whole register is 20 μA. As the temperature of an IC chip in which the register is provided becomes higher, the power consumption becomes larger and the OFF-state current of the register becomes several milliamperes.

In order to suppress power consumption, a method for stopping the supply of a power supply potential to a register has been suggested. In the method, in the case where data needs to be held for a long time in a register, the data is temporally transferred to a non-volatile memory device provided on the periphery of the register. This is because the register is a volatile memory device in which data is erased when the supply of a power supply potential is stopped. However, since such a non-volatile memory device is mainly formed using a magnetic element or a ferroelectric, the manufacturing process is complex.

In the case where the power supply is stopped for a long time, the data is transferred to a memory device such as a hard disk or a flash memory and then the power supply can be stopped; however, in the case where the power supply is stopped for a short time, such a memory device is not suitable because a time for placing the data back is needed.

In view of the above-described problems, an object of one embodiment of the present invention is to provide a memory device which does not need a complex manufacturing process and whose power consumption can be suppressed, a semiconductor device including the memory device, and an electronic device including the memory device. In particular, the object of one embodiment of the present invention is to provide a memory device whose power consumption can be suppressed by stopping a power supply for a short time, a semiconductor device including the memory device, and an electronic device including the memory device.

In a memory element including a logic element by which the phase of an input signal is inverted and the signal is output (hereinafter, referred to as a phase-inversion element) such as an inverter or a clocked inverter, a capacitor which holds data and a switching element which controls storing and releasing charge in the capacitor are provided. For the switching element, a transistor including an oxide semiconductor in a channel formation region is used.

Specifically, the memory element at least includes two phase-inversion elements, a capacitor, and a switching element which controls storing and releasing of charge in the capacitor. A signal including data that is input to the memory element is supplied to an input terminal of a first phase-inversion element. An output terminal of the first phase-inversion element is connected to an input terminal of a second phase-inversion element. An output terminal of the second phase-inversion element is connected to the input terminal of the first phase-inversion element. A potential of the output terminal of the first phase-inversion element or the input terminal of the second phase-inversion element is output to a memory element or another circuit of a subsequent stage as a signal.

Each of the phase-inversion elements has a structure in which at least one p-channel transistor and at least one n-channel transistor whose gate electrodes are connected to each other are connected in series between a first node and a second node.

The capacitor is connected to a node to which the potential of the above-described signal is supplied via the switching element so as to store the data of the signal that is input to the memory element as needed.

In the state where a power supply voltage is applied between the first node and the second node, when the signal including the data is input to the input terminal of the first phase-inversion element, the data is held in the first phase-inversion element and the second phase-inversion element. In the case where the application of the power supply voltage between the first node and the second node is stopped, before the application of the power supply voltage is stopped, the above-described switching element is turned on and the data of the signal is stored in the capacitor. With the above-described structure, even when the application of the power supply voltage to at least one of the phase-inversion elements is stopped, data can be held in the memory element.

The channel formation region of the transistor used for the above-described switching element includes a highly-purified oxide semiconductor, and therefore the transistor has a characteristic of extremely low OFF-state current.

The oxide semiconductor is a metal oxide showing semiconductor characteristics, and has high mobility obtained by microcrystalline or polycrystalline silicon, and uniform element characteristics obtained by amorphous silicon. In addition, an oxide semiconductor (a purified OS) which is highly-purified by reduction of impurities such as moisture or hydrogen which serve as electron donors (donors) is an i-type semiconductor (an intrinsic semiconductor) or extremely close to an i-type semiconductor (a substantially i-type semiconductor). Specifically, impurities such as moisture or hydrogen contained in the oxide semiconductor are removed so that the value of the hydrogen concentration in the oxide semiconductor measured by secondary ion mass spectrometry (SIMS) is less than or equal to 5×10¹⁹/cm³, preferably less than or equal to 5×10¹⁸/cm³, more preferably less than or equal to 5×10¹⁷/cm³, even more preferably less than 1×10¹⁶/cm³. With the above-described structure, the carrier density of an oxide semiconductor film, which can be measured by Hall effect measurement, can be less than 1×10¹⁴ cm³, preferably less than 1×10¹² cm⁻³, more preferably less than 1×10¹¹ cm⁻³ that is less than or equal to measurement limit. That is, the carrier density of the oxide semiconductor film can be extremely close to zero. Furthermore, the band gap of the oxide semiconductor is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, more preferably greater than or equal to 3 eV. Thus, with the use of the oxide semiconductor film which is highly purified by sufficiently reducing the concentration of impurities such as moisture or hydrogen, the OFF-state current of the transistor can be reduced.

The analysis of the hydrogen concentration in the oxide semiconductor film and a conductive film is noted here. The hydrogen concentration in the oxide semiconductor film and a conductive film is measured by secondary ion mass spectrometry (SIMS). Because of the principle of the SIMS analysis, it is known to be difficult to obtain accurate data in the proximity of a surface of a sample or in the proximity of an interface between stacked films formed using different materials. Thus, in the case where the distribution of the hydrogen concentration in the thickness direction of a film is analyzed by SIMS, the average value of the hydrogen concentration in a region of the film where almost the same value can be obtained without significant variation is employed as the hydrogen concentration. Also, when the thickness of the film is small, a region where almost the same value can be obtained cannot be found in some cases owing to the influence of the hydrogen concentration of an adjacent film. In this case, the maximal value or the minimal value of the hydrogen concentration in a region of the film is employed as the hydrogen concentration in the film. Furthermore, in the case where a mountain-shaped peak indicating the maximal value and a valley-shaped peak indicating the minimal value do not exist in any region of the film, the value of the inflection point is employed as the hydrogen concentration.

Note that it is known that the oxide semiconductor film formed by sputtering or the like contains a large amount of impurities such as moisture or hydrogen. Moisture or hydrogen easily forms a donor level and thus serves as impurities in the oxide semiconductor. In one embodiment of the present invention, in order to reduce impurities such as moisture or hydrogen in the oxide semiconductor film, the oxide semiconductor film is subjected to heat treatment in a reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a rare gas, or the like, an oxygen gas atmosphere, or an ultra dry air atmosphere (the moisture amount is 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, more preferably 10 ppb or less, in the case where measurement is performed by a dew point meter in a cavity ring down laser spectroscopy (CRDS) method). The temperature range of the heat treatment is preferably higher than or equal to 500° C. and lower than or equal to 850° C. (or a temperature lower than or equal to the strain point of a glass substrate), more preferably higher than or equal to 550° C. and lower than or equal to 750° C. Note that this heat treatment is performed at a temperature not exceeding the allowable temperature limit of the substrate to be used. An effect of elimination of moisture or hydrogen by the heat treatment has been confirmed by thermal desorption spectrometry (TDS).

Heat treatment in a furnace or a rapid thermal annealing method (RTA method) is used for the heat treatment. As the RTA method, a method using a lamp light source or a method in which short-time heat treatment is performed on a substrate that is transferred into a heated gas can be employed. With the use of the RTA method, the time necessary for the heat treatment can be shorter than 0.1 hour.

Specifically, the transistor including the highly-purified oxide semiconductor film that is highly-purified by the above-described heat treatment as an active layer has extremely low OFF-state current. Specifically, even when an element has a channel width (W) of 1×10⁶ μm and a channel length (L) of 10 μm, OFF-state current (drain current when the voltage between a gate electrode and a source electrode is less than or equal to 0 V) can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between the source electrode and the drain electrode of from 1 V to 10 V. Therefore, OFF-state current density corresponding to a numerical value which is calculated by dividing the OFF-state current by the channel width of the transistor is less than or equal to 100 zA/μm. According to measurement, it is further found that a transistor can have a much lower OFF-state current density, which is from 10 zA/μm to 100 zA/μm, at a voltage between a source electrode and a drain electrode of the transistor of 3 V. In the measurement, a transistor including a highly-purified oxide semiconductor film and a 100-nm-thick gate insulating film was used for a switching element which stores charge of a storage capacitor, and the OFF-state current of the transistor was measured by a change in the amount of charge in the storage capacitor per a unit hour. Thus, in the memory device which is one embodiment of the present invention, the OFF-state current density of the transistor including the highly-purified oxide semiconductor film as an active layer can be less than or equal to 100 zA/μm, preferably less than or equal to 10 zA/μm, more preferably less than or equal to 1 zA/μm. Accordingly, the transistor including the highly-purified oxide semiconductor film as an active layer has far lower OFF-state current than a transistor including silicon having crystallinity.

In addition, a transistor including a highly-purified oxide semiconductor shows almost no temperature dependence of OFF-state current. One of the reasons is that the conductivity becomes extremely close to that of an intrinsic semiconductor, and that the Fermi level is located in the middle of the forbidden band because the oxide semiconductor is highly purified by removal of impurities which serve as electron donors (donors) in the oxide semiconductor. Another reason is that the oxide semiconductor has an energy gap of greater than or equal to 3 eV and includes very few thermally excited carriers. Still another reason for almost no temperature dependence is that the source electrode and the drain electrode are in a degenerated state. The operation of the transistor is mostly performed by carriers injected to the oxide semiconductor from the degenerated source electrode, and the carrier density does not have temperature dependence; therefore, the temperature dependence of OFF-state current is unlikely to be shown.

By using a transistor having the above-described structure as a switching element which holds charge stored in a capacitor, leakage current from the capacitor can be prevented; therefore, without application of a power supply voltage, data is not erased but can be held. In the period when the data is held in the capacitor, the power supply voltage is not necessarily applied to at least one of the phase-inversion elements; as a result, surplus power consumption due to the OFF-state current of the transistor used for the phase-inversion element can be reduced, and the power consumption of the memory device and further the whole semiconductor device including the memory device can be suppressed to be low.

Note that in the transistor used in the phase-inversion element, a semiconductor other than an oxide semiconductor can be used, such as amorphous silicon, microcrystalline silicon, polycrystalline silicon, single crystal silicon, amorphous germanium, microcrystalline germanium, polycrystalline germanium, or single crystal germanium. In addition, in the above-described transistor, a thin semiconductor film or a bulk semiconductor substrate may be used. As long as a p-channel transistor including an oxide semiconductor film can be manufactured, all of the transistors in the memory element can include the oxide semiconductor film as an active layer, so that the process can be simplified.

In one embodiment of the present invention, the application of a power supply voltage to a memory element can be controlled with the use of a transistor including an oxide semiconductor in a channel formation region. As described above, the transistor including an oxide semiconductor in a channel formation region has a band gap of 3.0 eV to 3.5 eV, which is about three times as large as that of silicon. Since the transistor including an oxide semiconductor in a channel formation region has a high withstand voltage, by controlling the application of a power supply voltage to the memory element with the use of the transistor, the reliability of a semiconductor device can be increased.

As the oxide semiconductor, an oxide of four metal elements such as an In—Sn—Ga—Zn—O-based oxide semiconductor, an oxide of three metal elements such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxide semiconductor, an oxide of two metal elements such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxide semiconductor, an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, a Zn—O-based oxide semiconductor, or the like can be used. Note that in this specification, for example, an In—Sn—Ga—Zn—O-based oxide semiconductor means a metal oxide containing indium (In), tin (Sn), gallium (Ga), and zinc (Zn), and there is no particular limitation on the stoichiometric composition ratio. The above oxide semiconductors may include silicon.

Alternatively, oxide semiconductors can be represented by the chemical formula, InMO₃(ZnO)_(m) (m>0). Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co.

With one embodiment of the present invention, it is possible to provide a memory device that can suppress power consumption and a semiconductor device including the memory device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a memory element.

FIG. 2 is a circuit diagram of the memory element.

FIG. 3 is a circuit diagram of a memory element.

FIG. 4 is a circuit diagram of the memory element.

FIG. 5 is a circuit diagram of a memory element.

FIG. 6 is a circuit diagram of the memory element.

FIGS. 7A to 7E are views illustrating a method for manufacturing a memory device.

FIGS. 8A to 8D are views illustrating the method for manufacturing a memory device.

FIGS. 9A and 9B are views illustrating the method for manufacturing a memory device.

FIGS. 10A to 10C are views illustrating the method for manufacturing a memory device.

FIGS. 11A and 11B are views illustrating the method for manufacturing a memory device.

FIGS. 12A to 12C are cross-sectional views of memory devices.

FIGS. 13A and 13B are views illustrating structures of memory devices.

FIGS. 14A and 14B are a cross-sectional view and a top view of a transistor.

FIGS. 15A to 15E are views illustrating a method for manufacturing a memory device.

FIGS. 16A and 16B are circuit diagrams of a conventional memory element.

FIG. 17 is a block diagram of a CPU in which a memory device is used.

FIGS. 18A to 18F are views illustrating structures of electronic devices.

FIG. 19 is a cross-sectional view of a transistor including an oxide semiconductor.

FIG. 20 is an energy band diagram (a schematic diagram) along A-A′ section illustrated in FIG. 19.

FIG. 21A is a view illustrating a state where a positive voltage (V_(G)>0) is applied to a gate electrode (GE) and FIG. 21B is a view illustrating a state where a negative voltage (V_(G)<0) is applied to the gate electrode (GE).

FIG. 22 is a view showing relation between the vacuum level and the work function (φ_(M)) of a metal, and between the vacuum level and the electron affinity (χ) of an oxide semiconductor.

FIG. 23 is a cross-sectional view of a memory device.

FIG. 24 is a timing chart showing the operation of a memory device.

FIG. 25 is a timing chart showing the operation of a memory device.

FIG. 26 is a timing chart showing the operation of a memory device.

FIG. 27 is a timing chart showing the operation of a memory device.

FIG. 28 is a timing chart showing the operation of a memory device.

FIG. 29 is a timing chart showing the operation of a memory device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the scope and spirit of the present invention. Accordingly, the invention should not be construed as being limited to the description of the embodiments below.

The present invention includes in its category the following various kinds of semiconductor devices in which a memory device can be used: integrated circuits such as microprocessors, image processing circuits, digital signal processors (DSPs), and large scale integrated circuits (LSIs) including microcontrollers, RF tags, and semiconductor display devices. In addition, the semiconductor display devices include the following in its category: liquid crystal display devices, light-emitting devices in which a light-emitting element typified by an organic light-emitting element (OLED) is provided for each pixel, electronic papers, digital micromirror devices (DMDs), plasma display panels (PDPs), field emission displays (FEDs), and other semiconductor display devices in which a circuit element using a semiconductor film is included in a driver circuit.

Embodiment 1

A memory device which is one embodiment of the present invention includes one or a plurality of memory elements capable of storing 1-bit data. In FIG. 1, an example of a circuit diagram of a memory element included in a memory device of the present invention is illustrated. A memory element 100 illustrated in FIG. 1 at least includes a first phase-inversion element 101 and a second phase-inversion element 102 by which the phase of an input signal is inverted and the signal is output, a switching element 103, a switching element 104, a capacitor 105, and a capacitor switching element 106.

A signal IN including data that is input to the memory element 100 is supplied to an input terminal of the first phase-inversion element 101 via the switching element 103. An output terminal of the first phase-inversion element 101 is connected to an input terminal of the second phase-inversion element 102. An output terminal of the second phase-inversion element 102 is connected to the input terminal of the first phase-inversion element 101 via the switching element 104. A potential of the output terminal of the first phase-inversion element 101 or the input terminal of the second phase-inversion element 102 is output to a memory element or another circuit of a subsequent stage as a signal OUT.

Note that in FIG. 1, an example in which inverters are used as the first phase-inversion element 101 and the second phase-inversion element 102 is illustrated; however, a clocked inverter can also be used as the first phase-inversion element 101 or the second phase-inversion element 102 besides the inverter.

The capacitor 105 is connected to an input terminal of the memory element 100, i.e., a node to which a potential of the signal IN is supplied, via the switching element 103 and the capacitor switching element 106 so that the data of the signal IN that is input to the memory element 100 can be stored as needed. Specifically, the capacitor 105 is a condenser including a dielectric between a pair of electrodes. One of the electrodes is connected to the input terminal of the first phase-inversion element 101 via the capacitor switching element 106. The other of the electrodes is connected to a node to which a low-level power supply potential VSS or a fixed potential such as a ground potential is supplied.

For the capacitor switching element 106, a transistor including a highly-purified oxide semiconductor in a channel formation region is used.

Note that the memory element 100 may further include another circuit element such as a diode, a resistor, or an inductance, as needed.

Next, an example of a more specific circuit diagram of the memory element of FIG. 1 is illustrated in FIG. 2. The memory element 100 illustrated in FIG. 2 includes the first phase-inversion element 101, the second phase-inversion element 102, the switching element 103, the switching element 104, the capacitor 105, and the capacitor switching element 106. The connection structure of these circuit elements are the same as that in FIG. 1.

The first phase-inversion element 101 in FIG. 2 has a structure in which a p-channel transistor 107 and an n-channel transistor 108 whose gate electrodes are connected to each other are connected in series between a first node to which a high-level power supply potential VDD is supplied and a second node to which a low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 107 is connected to the first node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 108 is connected to the second node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 107 is connected to a drain electrode of the n-channel transistor 108, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the first phase-inversion element 101. In addition, potentials of the gate electrode of the p-channel transistor 107 and the gate electrode of the n-channel transistor 108 can be regarded as a potential of the input terminal of the first phase-inversion element 101.

The second phase-inversion element 102 in FIG. 2 has a structure in which a p-channel transistor 109 and an n-channel transistor 110 whose gate electrodes are connected to each other are connected in series between the first node to which the high-level power supply potential VDD is supplied and the second node to which the low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 109 is connected to the first node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 110 is connected to the second node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 109 is connected to a drain electrode of the n-channel transistor 110, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the second phase-inversion element 102. In addition, potentials of the gate electrode of the p-channel transistor 109 and the gate electrode of the n-channel transistor 110 can be regarded as a potential of the input terminal of the second phase-inversion element 102.

In FIG. 2, the case where a transistor is used for the switching element 103 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 1 supplied to a gate electrode thereof. In addition, the case where a transistor is used for the switching element 104 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 2 supplied to a gate electrode thereof.

Note that in FIG. 2, a structure in which each of the switching element 103 and the switching element 104 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the switching element 103 or the switching element 104 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the switching element 103 or the switching element 104, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

In FIG. 2, a transistor including an oxide semiconductor in a channel formation region is used for the capacitor switching element 106, and the switching of the transistor is controlled by a signal Sig 3 supplied to a gate electrode thereof. Since the transistor used for the capacitor switching element 106 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current is extremely low as described above.

Note that in FIG. 2, a structure in which the capacitor switching element 106 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the capacitor switching element 106 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the capacitor switching element 106, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

Note that in this specification, the state in which the transistors are connected to each other in series means the state in which only one of a source electrode and a drain electrode of a first transistor is connected to only one of a source electrode and a drain electrode of a second transistor. In addition, the state in which the transistors are connected to each other in parallel means the state in which one of a source electrode and a drain electrode of a first transistor is connected to one of a source electrode and a drain electrode of a second transistor and the other of the source electrode and the drain electrode of the first transistor is connected to the other of the source electrode and the drain electrode of the second transistor.

In one embodiment of the present invention, at least a transistor used for a switching element in the capacitor switching element 106 may include a highly-purified oxide semiconductor in a channel formation region. Accordingly, a transistor used for the first phase-inversion element 101, the second phase-inversion element 102, the switching element 103, or the switching element 104 can include a semiconductor other than an oxide semiconductor, such as amorphous silicon, microcrystalline silicon, polycrystalline silicon, single crystal silicon, amorphous germanium, microcrystalline germanium, polycrystalline germanium, or signal crystal germanium. In addition, in the above-described transistor, a thin semiconductor film or a bulk semiconductor substrate may be used. As long as a p-channel transistor including an oxide semiconductor film can be manufactured, all of the transistors in the memory element can include the oxide semiconductor film as an active layer, so that the process can be simplified.

Note that “connection” in this specification means electrical connection and corresponds to the state in which current, voltage, or potential can be supplied, applied, or conducted. Therefore, the state of connection does not always mean a state of direct connection, but includes in its category a state of indirect connection via a circuit element such as a wiring, a resistor, a diode, or a transistor, in which current, voltage, or potential can be supplied, applied, or conducted.

Note also that even when a circuit diagram shows independent components as if they are connected to each other, there is a case in which one conductive film has functions of a plurality of components such as a case in which part of a wiring also functions as an electrode. The “connection” in this specification includes in its category such a case in which one conductive film has functions of a plurality of components.

The “source electrode” and the “drain electrode” included in the transistor interchange with each other depending on the polarity of the transistor or difference between the levels of potentials supplied to the respective electrodes. In general, in an re-channel transistor, an electrode to which a lower potential is supplied is called a source electrode, and an electrode to which a higher potential is supplied is called a drain electrode. Further, in a p-channel transistor, an electrode to which a lower potential is supplied is called a drain electrode, and an electrode to which a higher potential is supplied is called a source electrode. In this specification, for convenience, although connection relation of the transistor is described assuming that the source electrode and the drain electrode are fixed in some cases; however, actually, the names of the source electrode and the drain electrode interchange with each other depending on the relation between the potentials.

Next, an example of the operation of the memory element illustrated in FIG. 1 will be described.

First, in writing data, the switching element 103 is turned on, the switching element 104 is turned off, and the capacitor switching element 106 is turned off. Then, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby a power supply voltage is applied between the first node and the second node. A potential of the signal IN supplied to the memory element 100 is supplied to the input terminal of the first phase-inversion element 101 via the switching element 103, whereby the potential of the output terminal of the first phase-inversion element 101 is a phase-inverted potential of the signal IN. Then, the switching element 104 is turned on and the input terminal of the first phase-inversion element 101 is connected to the output terminal of the second phase-inversion element 102, whereby data is written into the first phase-inversion element 101 and the second phase-inversion element 102.

Next, in the case where the input data is held in the first phase-inversion element 101 and the second phase-inversion element 102, in the state where the switching element 104 remains in an ON state and the capacitor switching element 106 remains in an OFF state, the switching element 103 is turned off. By turning off the switching element 103, the input data is held in the first phase-inversion element 101 and the second phase-inversion element 102. At this time, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby the state in which the power supply voltage is applied between the first node and the second node is maintained.

The potential of the output terminal of the first phase-inversion element 101 reflects the data held in the first phase-inversion element 101 and the second phase-inversion element 102. Therefore, by reading out the potential, the data can be read out from the memory element 100.

Note that in order to reduce power consumption in holding the data, in the case where the input data is held in the capacitor 105, first, the switching element 103 is turned off, the switching element 104 is turned on, and the capacitor switching element 106 is turned on. Then, via the capacitor switching element 106, charge with an amount corresponding to the value of the data held in the first phase-inversion element 101 and the second phase-inversion element 102 is stored in the capacitor 105, whereby the data is written into the capacitor 105. After the data is stored in the capacitor 105, the capacitor switching element 106 is turned off, whereby the data stored in the capacitor 105 is held. After turning off the capacitor switching element 106, for example, the power supply potential VSS is supplied to each of the first node and the second node so that the nodes have equal potentials, and the application of the power supply voltage between the first node and the second node is stopped. Note that after the data is stored in the capacitor 105, the switching element 104 may be turned off.

In such a manner, in the case where the input data is held in the capacitor 105, the application of the power supply voltage between the first node and the second node is unnecessary; therefore, the OFF-state current flowing between the first node and the second node via the p-channel transistor 107 and the n-channel transistor 108 which are included in the first phase-inversion element 101, or via the p-channel transistor 109 and the n-channel transistor 110 which are included in the second phase-inversion element 102 can be extremely close to zero. As a result, power consumption due to the OFF-state current of the memory element in holding the data can be significantly reduced, and the power consumption of the memory device and further the whole semiconductor device including the memory device can be suppressed to be low.

Since the transistor used for the capacitor switching element 106 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current density can be less than or equal to 100 zA/μm, preferably less than or equal to 10 zA/μm, more preferably less than or equal to 1 zA/μm. Accordingly, the transistor including the highly-purified oxide semiconductor film as an active layer has far lower OFF-state current than a transistor including silicon having crystallinity. As a result, when the capacitor switching element 106 for which the transistor is used is in an OFF state, charge stored in the capacitor 105 is hardly released; therefore, the data is held.

In the case where the data stored in the capacitor 105 is read out, the switching element 103 is turned off. Then, the power supply potential VDD is again supplied to the first node and the power supply potential VSS is again supplied to the second node, whereby the power supply voltage is applied between the first node and the second node. Then, by turning on the capacitor switching element 106, the signal OUT having a potential that reflects the data can be read out from the memory element 100.

Next, FIG. 24 is an example of a timing chart in the case where the supply of the power supply potential VDD is not stopped in the circuit illustrated in FIG. 2. When the signal Sig 1 is set to a low level and the signal Sig 2 is set to a high level, the signal IN is blocked, and a feedback loop is formed, and the state can be held. When the signal Sig 1 is set to a high level again and the signal Sig 2 is a set to a low level, the signal IN is input and output via the first phase-inversion element 101. At this time, the signal Sig 3 is set to a low level.

FIG. 25 is an example of a timing chart in the case where the supply of the power supply potential VDD is stopped in the circuit of FIG. 2. When the signal Sig 1 is set to a low level and the signal Sig 2 is set to a high level, the signal IN is blocked, and a feedback loop is formed, and the state can be held. After that, the signal Sig 3 is set to a high level, whereby the capacitor 105 stores the data (in FIG. 25, this state is shown as a high level). Even when the supply of the power supply potential VDD is stopped after that, a potential of the capacitor 105 is held. After that, when the power supply potential VDD is supplied and the signal Sig 3 is set to a high level again, the potential of the capacitor 105 is output via the first phase-inversion element 101 (in FIG. 25, this state is shown as a low level).

Embodiment 2

In this embodiment, another example of a memory element included in a memory device of the present invention will be described. In FIG. 3, a circuit diagram of the memory element of this embodiment is illustrated as an example.

A memory element 200 illustrated in FIG. 3 at least includes a first phase-inversion element 201, a second phase-inversion element 202, and a third phase-inversion element 207 by which the phase of an input signal is inverted and the signal is output, a switching element 203, a switching element 204, a switching element 208, a switching element 209, a capacitor 205, and a capacitor switching element 206.

A signal IN including data that is input to the memory element 200 is supplied to an input terminal of the first phase-inversion element 201 via the switching element 203. An output terminal of the first phase-inversion element 201 is connected to an input terminal of the second phase-inversion element 202. An output terminal of the second phase-inversion element 202 is connected to the input terminal of the first phase-inversion element 201 via the switching element 204. A potential of the output terminal of the first phase-inversion element 201 or the input terminal of the second phase-inversion element 202 is output to a memory element or another circuit of a subsequent stage as a signal OUT via the switching element 208.

The capacitor 205 is connected to an input terminal of the memory element 200, i.e., a node to which a potential of the signal IN is supplied, via the capacitor switching element 206 so that the data of the signal IN that is input to the memory element 200 can be stored as needed. Specifically, the capacitor 205 is a condenser including a dielectric between a pair of electrodes. One of the electrodes is connected to the node to which the potential of the signal IN is supplied via the capacitor switching element 206. The other of the electrodes is connected to a node to which a low-level power supply potential VSS or a fixed potential such as a ground potential is supplied.

In addition, the one of the electrodes of the capacitor 205 is connected to an input terminal of the third phase-inversion element 207. A potential of an output terminal of the third phase-inversion element 207 is output via the switching element 209 to a memory element or another circuit of a subsequent stage as a signal OUT.

Note that in FIG. 3, an example in which inverters are used as the first phase-inversion element 201, the second phase-inversion element 202, and the third phase-inversion element 207 is illustrated; however, a clocked inverter can also be used as the first phase-inversion element 201, the second phase-inversion element 202, or the third phase-inversion element 207 besides the inverter.

For the capacitor switching element 206, a transistor including a highly-purified oxide semiconductor in a channel formation region is used.

Note that the memory element 200 may further include another circuit element such as a diode, a resistor, or an inductance, as needed.

Next, an example of a more specific circuit diagram of the memory element of FIG. 3 is illustrated in FIG. 4. The memory element 200 illustrated in FIG. 4 at least includes the first phase-inversion element 201, the second phase-inversion element 202, the third phase-inversion element 207, the switching element 203, the switching element 204, the switching element 208, the switching element 209, the capacitor 205, and the capacitor switching element 206. The connection structure of these circuit elements are the same as that in FIG. 3.

The first phase-inversion element 201 in FIG. 4 has a structure in which a p-channel transistor 210 and an n-channel transistor 211 whose gate electrodes are connected to each other are connected in series between a first node to which a high-level power supply potential VDD is supplied and a second node to which a low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 210 is connected to the first node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 211 is connected to the second node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 210 is connected to a drain electrode of the n-channel transistor 211, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the first phase-inversion element 201. In addition, potentials of the gate electrode of the p-channel transistor 210 and the gate electrode of the n-channel transistor 211 can be regarded as a potential of the input terminal of the first phase-inversion element 201.

The second phase-inversion element 202 in FIG. 4 has a structure in which a p-channel transistor 212 and an n-channel transistor 213 whose gate electrodes are connected to each other are connected in series between the first node to which the high-level power supply potential VDD is supplied and the second node to which the low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 212 is connected to the first node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 213 is connected to the second node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 212 is connected to a drain electrode of the n-channel transistor 213, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the second phase-inversion element 202. In addition, potentials of the gate electrode of the p-channel transistor 212 and the gate electrode of the n-channel transistor 213 can be regarded as a potential of the input terminal of the second phase-inversion element 202.

The third phase-inversion element 207 in FIG. 4 has a structure in which a p-channel transistor 214 and an n-channel transistor 215 whose gate electrodes are connected to each other are connected in series between a third node to which the high-level power supply potential VDD is supplied and a fourth node to which the low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 214 is connected to the third node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 215 is connected to the fourth node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 214 is connected to a drain electrode of the n-channel transistor 215, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the third phase-inversion element 207. In addition, potentials of the gate electrode of the p-channel transistor 214 and the gate electrode of the n-channel transistor 215 can be regarded as a potential of the input terminal of the third phase-inversion element 207.

Note that the first node and the third node may be electrically connected to each other as one node. Note also that the second node and the fourth node may be electrically connected to each other as one node.

In FIG. 4, the case where a transistor is used for the switching element 203 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 1 supplied to a gate electrode thereof. In addition, the case where a transistor is used for the switching element 204 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 2 supplied to a gate electrode thereof. Further, the case where a transistor is used for the switching element 209 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 4 supplied to a gate electrode thereof.

Note that in FIG. 4, a structure in which each of the switching element 203, the switching element 204, and the switching element 209 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the switching element 203, the switching element 204, or the switching element 209 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the switching element 203, the switching element 204, or the switching element 209, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

In FIG. 4, a transistor including an oxide semiconductor in a channel formation region is used for the capacitor switching element 206, and the switching of the transistor is controlled by a signal Sig 3 supplied to a gate electrode thereof. Since the transistor used for the capacitor switching element 206 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current is extremely low as described above.

Note that in FIG. 4, a structure in which the capacitor switching element 206 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the capacitor switching element 206 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the capacitor switching element 206, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

In one embodiment of the present invention, at least a transistor used for a switching element in the capacitor switching element 206 may include a highly-purified oxide semiconductor in a channel formation region. Accordingly, a transistor used for the first phase-inversion element 201, the second phase-inversion element 202, the third phase-inversion element 207, the switching element 203, the switching element 204, the switching element 208, or the switching element 209 can include a semiconductor other than an oxide semiconductor, such as amorphous silicon, microcrystalline silicon, polycrystalline silicon, single crystal silicon, amorphous germanium, microcrystalline germanium, polycrystalline germanium, or signal crystal germanium. In addition, in the above-described transistor, a thin semiconductor film or a bulk semiconductor substrate may be used. As long as a p-channel transistor including an oxide semiconductor film can be manufactured, all of the transistors in the memory element can include the oxide semiconductor film as an active layer, so that the process can be simplified.

Next, an example of the operation of the memory element illustrated in FIG. 3 will be described.

First, in writing data, the switching element 203 is turned on, the switching element 204 is turned off, the switching element 208 is turned off, the switching element 209 is turned off, and the capacitor switching element 206 is turned on. Then, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby a power supply voltage is applied between the first node and the second node. A potential of the signal IN supplied to the memory element 200 is supplied to the input terminal of the first phase-inversion element 201 via the switching element 203, whereby the potential of the output terminal of the first phase-inversion element 201 is a phase-inverted potential of the signal IN. Then, the switching element 204 is turned on and the input terminal of the first phase-inversion element 201 is connected to the output terminal of the second phase-inversion element 202, whereby data is written into the first phase-inversion element 201 and the second phase-inversion element 202.

In writing the data, via the capacitor switching element 206, charge with an amount corresponding to the value of the data of the signal IN is stored in the capacitor 205, whereby the data is written also into the capacitor 205.

Note that in writing the data, application of a power supply voltage between the third node and the fourth node is unnecessary. Therefore, for example, the power supply potential VSS is supplied to each of the third node and the fourth node so that the nodes have equal potentials.

Next, in the case where the input data is held in the first phase-inversion element 201 and the second phase-inversion element 202, in the state where the switching element 204 remains in an ON state, the switching element 208 remains in an OFF state, and the switching element 209 remains in an OFF state, the switching element 203 is turned off and the capacitor switching element 206 is turned off. By turning off the switching element 203, the input data is held in the first phase-inversion element 201 and the second phase-inversion element 202. At this time, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby the state in which the power supply voltage is applied between the first node and the second node is maintained.

In addition, by turning off the capacitor switching element 206, the data written into the capacitor 205 is also held.

The potential of the output terminal of the first phase-inversion element 201 reflects the data held in the first phase-inversion element 201 and the second phase-inversion element 202. Therefore, by reading out the potential by turning on the switching element 208, the data can be read out from the memory element 200.

Note that in order to reduce power consumption in holding the data, in the case where the input data is held only in the capacitor 205, for example, the power supply potential VSS is supplied to each of the first node and the second node so that the nodes have equal potentials, and the application of the power supply voltage between the first node and the second node is stopped. When the application of the power supply voltage between the first node and the second node is stopped, the data held in the first phase-inversion element 201 and the second phase-inversion element 202 are erased, but the data written in the capacitor 205 remains to be held.

In such a manner, in the case where the input data is held in the capacitor 205, the application of the power supply voltage between the first node and the second node is unnecessary; therefore, via the p-channel transistor 210 and the n-channel transistor 211 which are included in the first phase-inversion element 201, or via the p-channel transistor 212 and the n-channel transistor 213 which are included in the second phase-inversion element 202, the OFF-state current flowing between the first node and the second node can be extremely close to zero. As a result, power consumption due to the OFF-state current of the memory element in holding the data can be significantly reduced, and the power consumption of the memory device and further the whole semiconductor device including the memory device can be suppressed to be low.

In addition, in the case where the input data is held in the capacitor 205, the application of the power supply voltage between the third node and the fourth node is unnecessary. Therefore, via the p-channel transistor 214 and the n-channel transistor 215 which are included in the third phase-inversion element 207, the OFF-state current flowing between the third node and the fourth node can be extremely close to zero. As a result, power consumption due to the OFF-state current of the memory element in holding the data can be significantly reduced, and the power consumption of the memory device and further the whole semiconductor device including the memory device can be suppressed to be low.

Since the transistor used for the capacitor switching element 206 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current density can be less than or equal to 100 zA/μm, preferably less than or equal to 10 zA/μm, more preferably less than or equal to 1 zA/μm. Accordingly, the transistor including the highly-purified oxide semiconductor film as an active layer has far lower OFF-state current than a transistor including silicon having crystallinity. As a result, when the capacitor switching element 206 for which the transistor is used is in an OFF state, charge stored in the capacitor 205 is hardly released; therefore, the data is held.

In the case where the data stored in the capacitor 205 is read out, the power supply potential VDD is supplied to the third node and the power supply potential VSS is supplied to the fourth node, whereby the power supply voltage is applied between the third node and the fourth node. When the power supply voltage is applied between the third node and the fourth node, the output terminal of the third phase-inversion element 207 is supplied with a phase-inversion potential of the potential of the input terminal thereof. Note that the input terminal of the third phase-inversion element 207 is supplied with a potential having a level corresponding to the amount of charge stored in the capacitor 205; thus, a potential of the output terminal thereof reflects the data. Thus, by turning on the switching element 209, the signal OUT having a potential that reflects the data can be read out from the memory element 200.

FIG. 26 is an example of a timing chart in the case where the supply of the power supply potential VDD is not stopped in the circuit of FIG. 4. When the signal Sig 1 is set to a low level and the signal Sig 2 is set to a high level, the signal IN is blocked, and a feedback loop is formed, and the state can be held. When the signal Sig 1 is set to a high level again and the signal Sig 2 is a set to a low level, the signal IN is input and output via the first phase-inversion element 201. At this time, the signal Sig 3 and the signal Sig 4 are set to a low level, and a signal Sig 5 is set to a high level.

FIG. 27 is an example of a timing chart in the case where the supply of the power supply potential VDD is stopped in the circuit of FIG. 4. When the signal Sig 1 is set to a low level and the signal Sig 2 is set to a high level, the signal IN is blocked, and a feedback loop is formed, and the state can be held. On the other hand, the signal Sig 3 is set to a high level, whereby the capacitor 205 stores the data (in FIG. 27, this state is shown as a low level). Even when the supply of the power supply potential VDD is stopped after that, a potential of the capacitor 205 is held. After that, when the power supply potential VDD is supplied and the signal Sig 5 and the signal Sig 3 are set to a low level and the signal Sig 4 is set to a high level, the potential of the capacitor 205 is output via the third phase-inversion element 207 (in FIG. 27, this state is shown as a high level).

This embodiment can be implemented by being combined as appropriate with the above-described embodiment.

Embodiment 3

In this embodiment, another example of a memory element included in a memory device of the present invention will be described. In FIG. 5, a circuit diagram of the memory element of this embodiment is illustrated as an example.

A memory element 300 illustrated in FIG. 5 at least includes a first phase-inversion element 301 and a second phase-inversion element 302 by which the phase of an input signal is inverted and the signal is output, a switching element 303, a switching element 304, a capacitor 305, a capacitor switching element 306, a capacitor 307, and a capacitor switching element 308.

A signal IN including data that is input to the memory element 300 is supplied to an input terminal of the first phase-inversion element 301 via the switching element 303. An output terminal of the first phase-inversion element 301 is connected to an input terminal of the second phase-inversion element 302. An output terminal of the second phase-inversion element 302 is connected to the input terminal of the first phase-inversion element 301 via the switching element 304. A potential of the output terminal of the first phase-inversion element 301 or the input terminal of the second phase-inversion element 302 is output to a memory element or another circuit of a subsequent stage as a signal OUT.

The capacitor 305 is connected to an input terminal of the memory element 300, i.e., a node to which a potential of the signal IN is supplied, via the switching element 303 and the capacitor switching element 306 so that the data of the signal IN that is input to the memory element 300 can be stored as needed. Specifically, the capacitor 305 is a condenser including a dielectric between a pair of electrodes. One of the electrodes is connected to the input terminal of the first phase-inversion element 301 via the capacitor switching element 306. The other of the electrodes is connected to a node to which a low-level power supply potential VSS or a fixed potential such as a ground potential is supplied.

In a manner similar to that of the capacitor 305, the capacitor 307 is connected to an input terminal of the memory element 300, i.e., a node to which a potential of the signal IN is supplied, via the switching element 303, the first phase-inversion element 301, and the capacitor switching element 308 so that the data of the signal IN input to the memory element 300 can be stored as needed. Specifically, the capacitor 307 is a condenser including a dielectric between a pair of electrodes. One of the electrodes is connected to the output terminal of the first phase-inversion element 301 via the capacitor switching element 308. The other of the electrodes is connected to a node to which a low-level power supply potential VSS or a fixed potential such as a ground potential is supplied.

Note that in FIG. 5, an example in which inverters are used as the first phase-inversion element 301 and the second phase-inversion element 302 is illustrated; however, a clocked inverter can also be used as the first phase-inversion element 301 or the second phase-inversion element 302 besides the inverter.

For each of the capacitor switching element 306 and the capacitor switching element 308, a transistor including a highly-purified oxide semiconductor in a channel formation region is used.

Note that the memory element 300 may further include another circuit element such as a diode, a resistor, or an inductance, as needed.

Next, an example of a more specific circuit diagram of the memory element of FIG. 5 is illustrated in FIG. 6. The memory element 300 illustrated in FIG. 6 at least includes the first phase-inversion element 301, the second phase-inversion element 302, the switching element 303, the switching element 304, the capacitor 305, the capacitor switching element 306, the capacitor 307, and the capacitor switching element 308. The connection structure of these circuit elements are the same as that in FIG. 5.

The first phase-inversion element 301 in FIG. 6 has a structure in which a p-channel transistor 309 and an n-channel transistor 310 whose gate electrodes are connected to each other are connected in series between a first node to which a high-level power supply potential VDD is supplied and a second node to which a low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 309 is connected to the first node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 310 is connected to the second node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 309 is connected to a drain electrode of the n-channel transistor 310, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the first phase-inversion element 301. In addition, potentials of the gate electrode of the p-channel transistor 309 and the gate electrode of the n-channel transistor 310 can be regarded as a potential of the input terminal of the first phase-inversion element 301.

The second phase-inversion element 302 in FIG. 6 has a structure in which a p-channel transistor 311 and an n-channel transistor 312 whose gate electrodes are connected to each other are connected in series between the first node to which the high-level power supply potential VDD is supplied and the second node to which the low-level power supply potential VSS is supplied. Specifically, a source electrode of the p-channel transistor 311 is connected to the first node to which the power supply potential VDD is supplied, and a source electrode of the n-channel transistor 312 is connected to the second node to which the power supply potential VSS is supplied. In addition, a drain electrode of the p-channel transistor 311 is connected to a drain electrode of the n-channel transistor 312, and potentials of the two drain electrodes can be regarded as a potential of the output terminal of the second phase-inversion element 302. In addition, potentials of the gate electrode of the p-channel transistor 311 and the gate electrode of the n-channel transistor 312 can be regarded as a potential of the input terminal of the second phase-inversion element 302.

In FIG. 6, the case where a transistor is used for the switching element 303 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 1 supplied to a gate electrode thereof. In addition, the case where a transistor is used for the switching element 304 is illustrated as an example, and the switching of the transistor is controlled by a signal Sig 2 supplied to a gate electrode thereof.

Note that in FIG. 6, a structure in which each of the switching element 303 and the switching element 304 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the switching element 303 or the switching element 304 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the switching element 303 or the switching element 304, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

In FIG. 6, a transistor including an oxide semiconductor in a channel formation region is used for the capacitor switching element 306, and the switching of the transistor is controlled by a signal Sig 3 supplied to a gate electrode thereof. Since the transistor used for the capacitor switching element 306 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current is extremely low as described above.

In FIG. 6, a transistor including an oxide semiconductor in a channel formation region is used for the capacitor switching element 308, and the switching of the transistor is controlled by a signal Sig 4 supplied to a gate electrode thereof. Since the transistor used for the capacitor switching element 308 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current is extremely low as described above.

Note that in FIG. 6, a structure in which the capacitor switching element 306 or the capacitor switching element 308 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the capacitor switching element 306 or the capacitor switching element 308 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the capacitor switching element 306 or the capacitor switching element 308, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

In one embodiment of the present invention, at least a transistor used for a switching element in the capacitor switching element 306 or the capacitor switching element 308 may include a highly-purified oxide semiconductor in a channel formation region. Accordingly, a transistor used for the first phase-inversion element 301, the second phase-inversion element 302, the switching element 303, or the switching element 304 can include a semiconductor other than an oxide semiconductor, such as amorphous silicon, microcrystalline silicon, polycrystalline silicon, single crystal silicon, amorphous germanium, microcrystalline germanium, polycrystalline germanium, or signal crystal germanium. In addition, in the above-described transistor, a thin semiconductor film or a bulk semiconductor substrate may be used. As long as a p-channel transistor including an oxide semiconductor film can be manufactured, all of the transistors in the memory element can include the oxide semiconductor film as an active layer, so that the process can be simplified.

Next, an example of the operation of the memory element illustrated in FIG. 5 or FIG. 6 will be described.

First, in writing data, the switching element 303 is turned on, the switching element 304 is turned off, the capacitor switching element 306 is turned off, and the capacitor switching element 308 is turned off. Then, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby a power supply voltage is applied between the first node and the second node. A potential of the signal IN supplied to the memory element 300 is supplied to the input terminal of the first phase-inversion element 301 via the switching element 303, whereby the potential of the output terminal of the first phase-inversion element 301 is a phase-inverted potential of the signal IN. Then, the switching element 304 is turned on and the input terminal of the first phase-inversion element 301 is connected to the output terminal of the second phase-inversion element 302, whereby data is written into the first phase-inversion element 301 and the second phase-inversion element 302.

Next, in the case where the input data is held in the first phase-inversion element 301 and the second phase-inversion element 302, in the state where the switching element 304 remains in an ON state, the capacitor switching element 306 remains in an OFF state, and the capacitor switching element 308 remains in an OFF state, the switching element 303 is turned off. By turning off the switching element 303, the input data is held in the first phase-inversion element 301 and the second phase-inversion element 302. At this time, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby the state in which the power supply voltage is applied between the first node and the second node is maintained.

The potential of the output terminal of the first phase-inversion element 301 reflects the data held in the first phase-inversion element 301 and the second phase-inversion element 302. Therefore, by reading out the potential, the data can be read out from the memory element 300.

Note that in order to reduce power consumption in holding the data, in the case where the input data is held in the capacitor 305 and the capacitor 307, the switching element 303 is turned off, the switching element 304 is turned on, the capacitor switching element 306 is turned on, and the capacitor switching element 308 is turned on. Then, via the capacitor switching element 306, charge with an amount corresponding to the value of the data held in the first phase-inversion element 301 and the second phase-inversion element 302 is stored in the capacitor 305, whereby the data is written into the capacitor 305. In addition, via the capacitor switching element 308, charge with an amount corresponding to the value of the data held in the first phase-inversion element 301 and the second phase-inversion element 302 is stored in the capacitor 307, whereby the data is written into the capacitor 307. Note that the polarity of a voltage between a pair of electrodes included in the capacitor 305 and a voltage between a pair of electrodes included in the capacitor 307 are opposite to each other.

After the data is stored in the capacitor 305, the capacitor switching element 306 is turned off, whereby the data stored in the capacitor 305 is held. In addition, after the data is stored in the capacitor 307, the capacitor switching element 308 is turned off, whereby the data stored in the capacitor 307 is held. After turning off the capacitor switching element 306 and the capacitor switching element 308, for example, the power supply potential VSS is supplied to each of the first node and the second node so that the nodes have equal potentials, and the application of the power supply voltage between the first node and the second node is stopped.

In such a manner, in the case where the input data is held in the capacitor 305 and the capacitor 307, the application of the power supply voltage between the first node and the second node is unnecessary; therefore, via the p-channel transistor 309 and the re-channel transistor 310 which are included in the first phase-inversion element 301, or via the p-channel transistor 311 and the n-channel transistor 312 which are included in the second phase-inversion element 302, the OFF-state current flowing between the first node and the second node can be extremely close to zero. As a result, power consumption due to the OFF-state current of the memory element in holding the data can be significantly reduced, and the power consumption of the memory device and further the whole semiconductor device including the memory device can be suppressed to be low.

Since the transistor used for each of the capacitor switching element 306 and the capacitor switching element 308 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current density can be less than or equal to 100 zA/μm, preferably less than or equal to 10 zA/μm, more preferably less than or equal to 1 zA/μm. Accordingly, the transistor including the highly-purified oxide semiconductor film as an active layer has far lower OFF-state current than a transistor including silicon having crystallinity. As a result, when the capacitor switching element 306 for which the transistor is used is in an OFF state, charge stored in the capacitor 305 is hardly released; therefore, the data is held. In addition, when the capacitor switching element 308 for which the transistor is used is in an OFF state, charge stored in the capacitor 307 is hardly released; therefore, the data is held.

In the case where the data stored in the capacitor 305 and the capacitor 307 is read out, the power supply potential VDD is supplied to the first node and the power supply potential VSS is supplied to the second node, whereby the power supply voltage is applied between the first node and the second node. In this state, the capacitor switching element 306 is turned on. When the power supply voltage is applied between the first node and the second node, the output terminal of the first phase-reversing element 301 is supplied with a phase-reversed potential of the potential of the input terminal thereof. Note that the input terminal of the first phase-inversion element 301 is supplied with a potential having a level corresponding to the amount of charge stored in the capacitor 305; thus, a potential of the output terminal thereof reflects the data. In addition, by turning on the capacitor switching element 308, a potential having a level corresponding to the amount of charge stored in the capacitor 305 is supplied to the output terminal of the first phase-inversion element 301. Thus, the signal OUT having a potential that reflects the data can be read out from the memory element 300.

FIG. 28 is an example of a timing chart in the case where the supply of the power supply potential VDD is not stopped in the circuit of FIG. 6. When the signal Sig 1 is set to a low level and the signal Sig 2 is set to a high level, the signal IN is blocked, and a feedback loop is formed, and the state can be held. When the signal Sig 1 is set to a high level again and the signal Sig 2 is a set to a low level, the signal IN is input and output via the first phase-inversion element 301. At this time, the signal Sig 3 and the signal Sig 4 are set to a low level.

FIG. 29 is an example of a timing chart in the case where the supply of the power supply potential VDD is stopped in the circuit of FIG. 6. When the signal Sig 1 is set to a low level and the signal Sig 2 is set to a high level, the signal IN is blocked, and a feedback loop is formed, and the state can be held. After that, the signal Sig 3 and the signal Sig 4 are set to a high level, whereby the capacitor 305 and the capacitor 307 store the data. Even when the supply of the power supply potential VDD is stopped after that, potentials of the capacitor 305 and the capacitor 307 are held (in FIG. 29, the capacitor 305 has a high-level potential, and the capacitor 307 has a low-level potential). After that, when the power supply potential VDD is supplied and the signal Sig 3 and the signal Sig 4 are set to a high level again, the potential of the capacitor 305 is output via the first phase-inversion element 301, and the potential of the capacitor 307 is output (in FIG. 29, both the capacitor 305 and the capacitor 307 have a low-level potential).

This embodiment can be implemented by being combined as appropriate with any of the above-described embodiments.

Embodiment 4

A semiconductor device which is one embodiment of the present invention includes a transistor including silicon and a transistor including an oxide semiconductor. The transistor including silicon can be formed using a silicon wafer, an SOI (silicon on insulator) substrate, a thin silicon film over an insulating surface, or the like.

An SOI substrate can be manufactured using, for example, UNIBOND (registered trademark) typified by Smart Cut (registered trademark), epitaxial layer transfer (ELTRAN) (registered trademark), a dielectric separation method, plasma assisted chemical etching (PACE), separation by implanted oxygen (SIMOX), or the like.

A semiconductor film of silicon formed over a substrate having an insulating surface may be crystallized by a known technique. As the known technique of crystallization, a laser crystallization method using a laser beam and a crystallization method using a catalytic element are given. Alternatively, a crystallization method using a catalytic element and a laser crystallization method may be combined. In the case of using a heat-resistant substrate such as quartz, it is possible to combine any of the following crystallization methods: a thermal crystallization method with an electrically heated oven, a lamp anneal crystallization method with infrared light, a crystallization method with a catalytic element, and high temperature annealing method at about 950° C.

In addition, a semiconductor element manufactured using the above-described method may be transferred onto a flexible substrate formed of plastic or the like, so that a semiconductor device is formed. A variety of transfer methods can be used. Examples of the transfer methods include a method in which a metal oxide film is provided between the substrate and the semiconductor element, and the metal oxide film is embrittled by crystallization so that the semiconductor element is separated off and transferred; a method in which an amorphous silicon film containing hydrogen is provided between the substrate and the semiconductor element, and the amorphous silicon film is removed by laser beam irradiation or etching so that the semiconductor element is separated off from the substrate and transferred; a method in which the substrate, for which the semiconductor element is provided, is removed by mechanical cutting or etching by a solution or a gas so that the semiconductor element is cut off from the substrate and transferred; and the like.

In this embodiment, a structure of a semiconductor device and a method for manufacturing the semiconductor device will be described by giving an example in which with the use of an SOI (silicon on insulator) substrate, the transistor including silicon is manufactured and then the transistor including an oxide semiconductor is manufactured.

First, as illustrated in FIG. 7A, a bond substrate 500 is cleaned, and then, an insulating film 501 is formed over a surface of the bond substrate 500.

As the bond substrate 500, a single crystal semiconductor substrate formed using silicon can be used. Alternatively, the bond substrate 500 may be a semiconductor substrate formed using silicon having crystal lattice distortion, silicon germanium obtained by adding germanium to silicon, or the like.

Note that in a single crystal semiconductor substrate used for the bond substrate 500, the directions of crystal axes are preferably uniform; however, the substrate is not necessarily formed using perfect crystals in which a lattice defect such as a point defect, a line defect, or a plane defect is completely eliminated.

In addition, the shape of the bond substrate 500 is not limited to a circle, and the substrate can be processed into a shape other than a circle. The bond substrate 500 may be processed into, for example, a rectangular shape considering that the shape of a base substrate 503 to which the bond substrate 500 is attached later is generally a rectangle and that a light exposure region of a light exposure apparatus such as a reduced projection exposure apparatus is rectangular, and the like. The bond substrate 500 can be processed by cutting a circular single crystal semiconductor substrate that is commercially available.

The insulating film 501 may be a single insulating film or a stack of plural insulating films. Considering that a region containing impurities will be removed later, the thickness of the insulating film 501 is preferably greater than or equal to 15 nm and less than or equal to 500 nm.

As a film included in the insulating film 501, an insulating film containing silicon or germanium as its component such as a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, a germanium oxide film, a germanium nitride film, a germanium oxynitride film, or a germanium nitride oxide film can be used. Further, an insulating film containing a metal oxide such as aluminum oxide, tantalum oxide, or hafnium oxide; an insulating film containing a metal nitride such as aluminum nitride; an insulating film containing a metal oxynitride such as an aluminum oxynitride film; or an insulating film containing a metal nitride oxide such as an aluminum nitride oxide film can be used.

In this embodiment, an example in which silicon oxide formed by thermal oxidation of the bond substrate 500 is used as the insulating film 501 is described. Note that in FIG. 7A, the insulating film 501 is formed so as to cover the entire surface of the bond substrate 500; however, the insulating film 501 may be formed on at least one surface of the bond substrate 500.

Note that in this specification, an oxynitride is a substance containing oxygen and nitrogen so that the amount of oxygen is larger than that of nitrogen, whereas a nitride oxide is a substance containing oxygen and nitrogen so that the amount of nitrogen is larger than that of oxygen.

In the case where the insulating film 501 is formed by thermal oxidation of the surface of the bond substrate 500, dry oxidation in which oxygen having a small amount of moisture is used, thermal oxidation in which gas including a halogen such as hydrogen chloride is added to an oxygen atmosphere, or the like can be used as the thermal oxidation. In addition, wet oxidation such as pyrogenic oxidation in which hydrogen is burnt with oxygen to generate water, or water vapor oxidation in which high-purity water is heated at 100° C. or higher to generate water vapor and oxidation is performed with use of the water vapor may be used for forming the insulating film 501.

In the case where the base substrate 503 includes impurities which decrease the reliability of a semiconductor device, such as an alkali metal or an alkaline earth metal, the insulating film 501 preferably includes at least one layer of a barrier film that can prevent the impurities from diffusing from the base substrate 503 into a semiconductor film which is to be formed after separation. As the insulating film that can be used as the barrier film, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be given. The insulating film that is used as the barrier film is preferably formed to a thickness of 15 nm to 300 nm, for example. In addition, an insulating film which has a lower proportion of nitrogen than the barrier film, such as a silicon oxide film or a silicon oxynitride film may be formed between the barrier film and the bond substrate 500. The insulating film which has a lower proportion of nitrogen may be formed to a thickness of greater than or equal to 5 nm and less than or equal to 200 nm.

In the case of using silicon oxide for the insulating film 501, the insulating film 501 can be formed by a vapor deposition method such as thermal CVD, plasma CVD, normal pressure CVD, or bias ECRCVD with the use of a mixed gas of silane and oxygen, a mixed gas of TEOS (tetraethoxysilane) and oxygen, or the like. In this case, a surface of the insulating film 501 may be subjected to oxygen plasma treatment to increase the density. Meanwhile, in the case of using silicon nitride for the insulating film 501, the insulating film 501 can be formed by a vapor deposition method such as plasma CVD with the use of a mixed gas of silane and ammonia.

Alternatively, the insulating film 501 may be formed using silicon oxide by a chemical vapor deposition method with the use of an organosilane gas. As an organosilane gas, a silicon-containing compound such as tetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄), tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula: SiH(OC₂H₅)₃), or trisdimethylaminosilane (chemical formula: SiH(N(CH₃)₂)₃) can be used.

With the use of an organosilane gas for the source gas, a silicon oxide film with a flat surface can be formed at a process temperature of 350° C. or lower. Alternatively, the insulating film can be formed by a thermal CVD method using a low temperature oxide (LTO) which is formed at a heating temperature of higher than or equal to 200° C. and lower than or equal to 500° C. LTO can be formed by using monosilane (SiH₄), disilane (Si₂H₆), or the like as a silicon source gas and using nitrogen dioxide (NO₂) or the like as an oxygen source gas.

For example, in the case of using TEOS and O₂ for the source gas to form the silicon oxide film as the insulating film 501, the condition may be set as follows: the flow rate of TEOS is 15 sccm, the flow rate of O₂ is 750 sccm, the deposition pressure is 100 Pa, the deposition temperature is 300° C., the RF output is 300 W, and the power source frequency is 13.56 MHz.

Note that an insulating film formed at a relatively low temperature, such as a silicon oxide film formed using organosilane or a silicon nitride oxide film formed at a low temperature, has a number of OH groups on its surface. Hydrogen bonding between the OH group and a water molecule forms a silanol group and bonds the base substrate and the insulating film at a low temperature. A siloxane bond, which is a covalent bond, is formed finally between the base substrate and the insulating film. The insulating film such as the above-described silicon oxide film formed using organosilane or the LTO formed at a relatively low temperature is suitable for bonding at a low temperature, as compared with a thermally oxidized film having no OH groups or having very few OH groups which is used in Smart Cut (registered trademark) or the like.

The insulating film 501 forms a bonding plane which is flat and hydrophilic over the surface of the bond substrate 500. Therefore, the average surface roughness R_(a) of the insulating film 501 is preferably less than or equal to 0.7 nm, more preferably less than or equal to 0.4 nm. The thickness of the insulating film 501 may be greater than or equal to 5 nm and less than or equal to 500 nm, preferably greater than or equal to 10 nm and less than or equal to 200 nm.

Next, as illustrated in FIG. 7B, the bond substrate 500 is irradiated with an ion beam including ions accelerated by an electric field through the insulating film 501 as indicated by arrows, whereby an embrittlement layer 502 having microvoids is formed in a region at a predetermined depth from the surface of the bond substrate 500. For example, the embrittlement layer means a layer which is locally embrittled by disorder of a crystal structure, and the state of the embrittlement layer depends on a means for forming the embrittlement layer. Note that there may be a case where a region ranging from one surface of the bond substrate to the embrittlement layer is embrittled to some extent; however, the embrittlement layer in this specification indicates a region at which separation is performed later and its vicinity.

The depth at which the embrittlement layer 502 is formed can be adjusted by the acceleration energy of the ion beam and the angle at which the ion beam enters. The embrittlement layer 502 can be formed at the same or substantially the same depth as the average penetration depth of the ions. The thickness of a semiconductor film 504 which will be separated from the bond substrate 500 is determined by the depth at which the ions are implanted. The depth at which the embrittlement layer 502 is formed can be set in the range of, for example, greater than or equal to 50 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 200 nm.

The ions are implanted to the bond substrate 500 desirably by an ion doping method in which mass separation is not performed because the cycle time can be shortened; however, the present invention may employ an ion implantation method in which mass separation is performed.

When hydrogen (H₂) is used for a source gas, H⁺, H₂ ⁺, and H₃ ⁺ can be produced by exciting a hydrogen gas. Proportions of ion species produced from the source gas can be changed by adjusting a plasma excitation method, the pressure of an atmosphere for generating plasma, the amount of supplied source gas, or the like. In the case where the ion implantation is performed by an ion doping method, it is preferable that H₃ ⁺ be contained at 50% or more, more preferably at 80% or more, with respect to the total amount of H⁺, H₂ ⁺, and H₃ ⁺ in the ion beam. When H₃ ⁺ is contained at 80% or higher, the proportion of H₂ ⁺ ions in the ion beam gets lower relatively, which results in smaller variation in the average penetration depth of the hydrogen ions contained in the ion beam. Consequently, the ion implantation efficiency improves and the cycle time can be shortened.

H₃ ⁺ has larger mass than H⁺ and H₂ ⁺. When the ion beam containing a higher proportion of H₃ ⁺ is compared with the ion beam containing a higher proportion of H⁺ and H₂ ⁺ the former can implant hydrogen into a shallower region of the bond substrate 500 than the latter even when the acceleration voltage at the time of doping is the same. Moreover, the former has a steep concentration distribution of hydrogen implanted into the bond substrate 500 in a thickness direction, therefore, the thickness of the embrittlement layer 502 itself can be smaller.

In the case of performing ion implantation by an ion doping method with the use of a hydrogen gas, the acceleration voltage is set to greater than or equal to 10 kV and less than or equal to 200 kV and the dose is set to greater than or equal to 1×10¹⁶ ions/cm² and less than or equal to 6×10¹⁶ ions/cm². Under this condition, the embrittlement layer 502 can be formed in a region at a depth of greater than or equal to 50 nm and less than or equal to 500 nm of the bond substrate 500, although depending on the ion species included in the ion beam and its proportion, and the film thickness of the insulating film 501.

For example, in the case where the bond substrate 500 is a single crystal silicon substrate and the insulating film 501 is formed using a 100-nm-thick thermal oxide film, a semiconductor film with a thickness of approximately 146 nm can be separated from the bond substrate 500 under the condition where the flow rate of 100% hydrogen gas, which is the source gas, is 50 sccm, the beam current density is 5 μA/cm², the acceleration voltage is 50 kV, and the dose is 2.0×10¹⁶ atoms/cm². Note that even when the condition at the time of adding hydrogen to the bond substrate 500 is not changed, the thickness of the insulating film 501 is made larger, whereby the thickness of the semiconductor film can be made smaller.

Helium (He) can alternatively be used as the source gas of the ion beam. Since most of the ion species produced by exciting helium are He⁺, He⁺ can be mainly implanted into the bond substrate 500 even by an ion doping method in which mass separation is not performed. Therefore, microvoids can be formed in the embrittlement layer 502 efficiently by an ion-doping method. In the case of performing ion implantation by an ion doping method using helium, the acceleration voltage can be greater than or equal to 10 kV and less than or equal to 200 kV, and the dose can be greater than or equal to 1×10¹⁶ ions/cm² and less than or equal to 6×10¹⁶ ions/cm².

A halogen gas such as a chlorine gas (Cl₂ gas) or a fluorine gas (F₂ gas) can be used as a source gas.

In the case of performing ion implantation on the bond substrate 500 by an ion doping method, impurities existing in an ion doping apparatus are implanted together with the ions to a processing object; therefore, there is a possibility that impurities such as S, Ca, Fe, and Mo exist on and near the surface of the insulating film 501. Therefore, a region on and near the surface of the insulating film 501 where the number of impurities is considered to be the largest may be removed by etching, polishing, or the like. Specifically, a region at a depth of 10 nm to 100 nm, preferably approximately 30 nm to 70 nm from the surface of the insulating film 501 may be removed. The etching may be performed by a dry etching method such as a reactive ion etching (RIE) method; for example, an inductively coupled plasma (ICP) etching method, an electron cyclotron resonance (ECR) etching method, a parallel plate (capacitive coupled type) etching method, a magnetron plasma etching method, a dual-frequency plasma etching method, a helicon wave plasma etching method, or the like can be used. For example, in the case of removing a region on and near a surface of a silicon nitride oxide film by an ICP etching method, the region can be removed to a depth of about 50 nm from the surface under the condition where the flow rate of CHF₃ as an etching gas is 7.5 sccm, the flow rate of He is 100 sccm, the reaction pressure is 5.5 Pa, the temperature of a lower electrode is 70° C., the RF (13.56 MHz) electric power applied to a coil-shaped electrode is 475 W, the electric power applied to the lower electrode (on bias side) is 300 W, and the etching time is about 10 seconds.

Instead of CHF₃, which is a fluorine-based gas, a chlorine-based gas such as Cl₂, BCl₃, SiCl₄, or CCl₄; another fluorine-based gas such as CF₄, SF₆, or NF₃; or O₂ can be used as appropriate for the etching gas. Moreover, an inert gas other than He may be added to the etching gas. For example, one or plural elements selected from Ne, Ar, Kr, and Xe can be used as the inert element which is added to the etching gas. In the case of removing a region on and near a surface of a silicon nitride oxide film by wet etching, an aqueous solution containing ammonium hydrogen fluoride, ammonium fluoride, or the like, may be used as an etchant. The polishing can be performed by CMP (chemical mechanical polishing), liquid jet polishing, or the like.

After the formation of the embrittlement layer 502, an extremely contaminated region on and near the surface of the insulating film 501 is removed by etching, polishing, or the like, whereby the amount of impurities which enter the semiconductor film 504 formed over the base substrate 503 can be suppressed. Moreover, in a semiconductor device which is completed finally, it is possible to prevent the impurities from causing decrease in reliability and decrease in electric characteristics of transistors, such as variation in threshold voltage or increase in leakage current.

In order to remove the impurities, it is possible to perform atomic beam or ion beam irradiation treatment, plasma treatment, or radical treatment. In the case of using an atomic beam or an ion beam, an inert gas neutral atom beam or an inert gas ion beam of argon or the like can be used.

Next, as illustrated in FIG. 7C, the bond substrate 500 and the base substrate 503 are attached to each other so that the insulating film 501 is interposed therebetween.

Note that before the attachment of the base substrate 503 and the bond substrate 500, surfaces for the attachment, i.e., a surface of the insulating film 501 formed over the bond substrate 500 and a surface of the base substrate 503 in this embodiment, are preferably subjected to surface treatment for increasing bonding strength between the insulating film 501 and the base substrate 503.

As examples of the surface treatment, wet treatment, dry treatment, and combination of wet treatment and dry treatment can be given. Different wet treatment or different dry treatment may be combined to be performed. Examples of the wet treatment include ozone treatment using ozone water (ozone water cleaning), ultrasonic cleaning such as megasonic cleaning, two-fluid cleaning (a method in which functional water such as pure water or hydrogenated water and a carrier gas such as nitrogen are sprayed together), cleaning with hydrochloric acid and a hydrogen peroxide solution, and the like. As examples of the dry treatment, inert gas neutral atomic beam treatment, inert gas ion beam treatment, ultraviolet treatment, ozone treatment, plasma treatment, plasma treatment with bias application, radical treatment, and the like can be given. By performing the above-described surface treatment, the hydrophilicity and cleanliness of the surfaces for the attachment can be increased. Thus, the bonding strength can be improved.

For the attachment, the base substrate 503 and the insulating film 501 formed over the bond substrate 500 are disposed in close contact with each other, and then, a pressure of approximately greater than or equal to 1 N/cm² and less than or equal to 500 N/cm², preferably greater than or equal to 11 N/cm² and less than or equal to 20 N/cm² is applied to part of the base substrate 503 and the bond substrate 500 which are superposed on each other. When the pressure is applied, bonding between the base substrate 503 and the insulating film 501 starts from the portion, which results in bonding of the entire surface where the base substrate 503 and the insulating film 501 are in close contact with each other.

The bonding is performed by van der Waals force or a hydrogen bonding, so that the bonding is firm even at room temperature. Note that since the above-described bonding can be performed at a low temperature, a variety of substrates can be used for the base substrate 503. For example, a variety of glass substrates for electronics industry, such as an alumino silicate glass substrate, a barium borosilicate glass substrate, or an aluminoborosilicate glass substrate, a quartz substrate, a ceramic substrate, a sapphire substrate, or the like can be used as the base substrate 503. Alternatively, as the base substrate 503, a semiconductor substrate formed of silicon, gallium arsenide, indium phosphide, or the like can be used. Further alternatively, the base substrate 503 may be a metal substrate including a stainless steel substrate. Substrates with coefficients of thermal expansion of greater than or equal to 25×10⁻⁷/° C. and less than or equal to 50×10⁻⁷/° C. (preferably, greater than or equal to 30×10⁻⁷/° C. and less than or equal to 40×10⁻⁷/° C.) and strain points of greater than or equal to 580° C. and less than or equal to 680° C. (preferably, greater than or equal to 600° C. and less than or equal to 680° C.) are preferably used as the glass substrate which serves as the base substrate 503. When the glass substrate is an alkali-free glass substrate, impurity contamination of semiconductor devices can be suppressed.

As the glass substrate, a mother glass substrate developed for production of liquid crystal panels can be used. As such a mother glass substrate, substrates having the following sizes are known: the third generation (550 mm×650 mm), the 3.5-th generation (600 mm×720 mm), the fourth generation (680 mm×880 mm, or 730 mm×920 mm), the fifth generation (1100 mm×1300 mm), the sixth generation (1500 mm×1850 mm), the seventh generation (1870 mm×2200 mm), the eighth generation (2200 mm×2400 mm), and the like. When an SOI substrate is manufactured with a large-area mother glass substrate used as the base substrate 503, a large-area SOI substrate can be obtained. Size increase of an SOI substrate can be realized by using a large substrate such as a mother glass substrate, as the base substrate 503. When the SOI substrate can have a larger area, a large number of chips such as ICs or LSIs can be manufactured by using one substrate. Thus, productivity can be dramatically increased.

Although there is no particular limitation on a substrate which can be used as the base substrate 503, it is necessary that the substrate have at least enough heat resistance to heat treatment performed later. For example, a glass substrate manufactured by a fusion method or a float method can be used. When the temperature of the heat treatment performed later is high, a substrate having a strain point of 730° C. or higher is preferably used as the glass substrate. As a glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. In general, by containing barium oxide (BaO) and boron oxide so that the amount of barium oxide is larger than that of boron oxide, a glass substrate that is heat-resistant and of more practical use can be obtained. Therefore, a glass substrate containing BaO and B₂O₃ so that the amount of BaO is larger than that of B₂O₃ is preferably used. Note that in order to avoid such defective bonding that is caused by the shrink, the base substrate 503 may be subjected to heat treatment in advance before the bonding step.

Moreover, an insulating film may be formed in advance over the base substrate 503. The base substrate 503 is not necessarily provided with an insulating film on its surface. However, the formation of the insulating film on the surface of the base substrate 503 can prevent impurities of the base substrate 503, such as an alkali metal and an alkaline earth metal, from entering the bond substrate 500. Moreover, in the case of forming the insulating film on the surface of the base substrate 503, the insulating film over the base substrate 503 is bonded to the insulating film 501; therefore, a wider variety of substrates can be used as the base substrate 503. In general, the upper temperature limits of substrates formed of flexible synthetic resins such as plastics tend to be low. However, as long as the substrates can resist process temperatures in manufacturing steps of a semiconductor element performed later, the substrates formed of such resins can be used as the base substrate 503 in the case of forming the insulating film over the base substrate 503. Examples of a plastic substrate include polyester typified by polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polycarbonate (PC), polyetheretherketone (PEEK), polysulfone (PSF), polyetherimide (PEI), polyarylate (PAR), polybutylene terephthalate (PBT), polyimide, acrylonitrile-butadiene-styrene resin, polyvinyl chloride, polypropylene, polyvinyl acetate, acrylic resin, and the like. In the case of forming the insulating film over the base substrate 503, the attachment of the base substrate 503 and the bond substrate 500 to each other is preferably performed after surface treatment is performed on the surface of this insulating film in a manner similar to that for the insulating film 501.

After the bond substrate 500 is attached to the base substrate 503, heat treatment is preferably performed in order to increase the bonding strength at the bonding interface between the base substrate 503 and the insulating film 501. This treatment is performed at a temperature where a crack is not generated in the embrittlement layer 502 and can be performed at a temperature in the range of higher than or equal to 200° C. and lower than or equal to 400° C. By attaching the bond substrate 500 to the base substrate 503 within this temperature range, the bonding strength between the base substrate 503 and the insulating film 501 can be made firm.

When the bonding plane is contaminated by dust or the like at the time of attaching the bond substrate 500 and the base substrate 503 to each other, the contaminated portion is not bonded. In order to avoid the contamination of the bonding plane, the attachment of the bond substrate 500 and the base substrate 503 is preferably performed in an airtight chamber. At the time of attaching the bond substrate 500 and the base substrate 503 to each other, the treatment chamber may have pressure reduced to approximately 5.0×10⁻³ Pa and the atmosphere of the bonding treatment may be cleaned.

Subsequently, heat treatment is performed whereby microvoids adjacent to each other in the embrittlement layer 502 are combined, so that the microvoids increase in volume. As a result, as illustrated in FIG. 7D, the semiconductor film 504 which is part of the bond substrate 500 is separated from the bond substrate 500 along the embrittlement layer 502. Since the insulating film 501 and the base substrate 503 are bonded to each other, the semiconductor film 504 which is separated from the bond substrate 500 is fixed to the base substrate 503. The heat treatment for separating the semiconductor film 504 from the bond substrate 500 is preferably performed at a temperature which does not exceed the strain point of the base substrate 503.

For this heat treatment, a rapid thermal annealing (RTA) apparatus, a resistive heating furnace, or a microwave heating apparatus can be used. For the RTA apparatus, a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. When a GRTA apparatus is used, the heating temperature can be set at a temperature of higher than or equal to 550° C. and lower than or equal to 650° C., and the treatment time can be set at greater than or equal to 0.5 minute and less than or equal to 60 minutes. When a resistance heating apparatus is used, the heat temperature can be set at higher than or equal to 200° C. and lower than or equal to 650° C. and the treatment time can be set at greater than or equal to 2 hours and less than or equal to 4 hours.

The heat treatment may be performed by dielectric heating with a high-frequency wave such as a microwave. The heat treatment by the dielectric heating can be performed by irradiating the bond substrate 500 with high frequency waves generated at a high-frequency generator, which is in the range of 300 MHz to 3 THz. Specifically, for example, irradiation with a microwave with a frequency of 2.45 GHz at 900 W is performed for 14 minutes to combine microvoids adjacent to each other in the embrittlement layer, whereby the bond substrate 500 can be split along the embrittlement layer finally.

A specific treatment method of heat treatment using a vertical furnace having resistive heating is described. The base substrate 503 to which the bond substrate 500 is attached is disposed on a boat of the vertical furnace and this boat is delivered in a chamber of the vertical furnace. In order to suppress oxidation of the bond substrate 500, the chamber is evacuated first such that a vacuum state is formed. The degree of vacuum is approximately 5×10⁻³ Pa. After a vacuum state is obtained, nitrogen is supplied to the chamber so that the chamber has a nitrogen atmosphere under atmospheric pressure. In this period, the heating temperature is increased to 200° C.

After making the chamber have a nitrogen atmosphere that is under atmospheric pressure, heating at 200° C. is performed for 2 hours. Then, the temperature is increased to 400° C. in 1 hour. After the state at a heating temperature of 400° C. becomes stable, the temperature is increased to 600° C. in 1 hour. After the state at a heating temperature of 600° C. becomes stable, heat at 600° C. is performed for 2 hours. Then, the heating temperature is decreased to 400° C. in 1 hour, and after 10 minutes to 30 minutes, the boat is carried out from the chamber. The base substrate 503 to which the bond substrate 500 and the semiconductor film 504 are attached and which is disposed on the boat is cooled in an air atmosphere.

The heat treatment using the above resistance heating furnace is performed by successively performing heat treatment for increasing the bonding strength between the insulating film 501 and the base substrate 503 and heat treatment for splitting the embrittlement layer 502. In the case of performing these two kinds of heat treatment in different apparatuses, for example, heat treatment is performed at 200° C. for 2 hours in a resistance heating furnace and then the base substrate 503 and the bond substrate 500 which are attached to each other are carried out from the furnace. Next, heat treatment is performed by an RTA apparatus at a process temperature of higher than or equal to 600° C. and lower than or equal to 700° C. for one minute to several hours, so that the bond substrate 500 is split along the embrittlement layer 502.

Note that in some cases, a periphery of the bond substrate 500 is not bonded to the base substrate 503. This is seemingly because the periphery of the bond substrate 500 is chamfered or has a curvature, so that the base substrate 503 and the insulating film 501 are not in close contact with each other or the embrittlement layer 502 is difficult to split at the periphery of the bond substrate 500. Another reason is that polishing such as CMP performed in manufacturing the bond substrate 500 is insufficient at the periphery of the bond substrate 500, so that a surface thereof is rougher at the periphery than at a center. Still another reason is that, in the case where a carrier or the like damages the periphery of the bond substrate 500 at the time of delivery of the bond substrate 500, the damage makes it difficult to bond the periphery to the base substrate 503. For these reasons, the semiconductor film 504 which is smaller than the bond substrate 500 is attached to the base substrate 503.

Note that the bond substrate 500 may be subjected to hydrogenation treatment before the bond substrate 500 is split. Hydrogenation treatment is performed, for example, at 350° C. for about 2 hours in a hydrogen atmosphere.

When a plurality of bond substrates 500 is attached to the base substrate 503, the plural bond substrates 500 may have different crystal plane orientation. The mobility of majority carriers in a semiconductor depends on crystal plane orientation. Therefore, the semiconductor film 504 may be formed by selecting as appropriate the bond substrate 500 which has crystal plane orientation suitable for a semiconductor element to be formed. For example, in a case of forming an n-type semiconductor element by using the semiconductor film 504, the mobility of majority carriers in the semiconductor element can be increased by forming the semiconductor film 504 which has a {100} plane. In contrast, for example, in the case of forming a p-type semiconductor element by using the semiconductor film 504, the mobility of majority carriers in the semiconductor element can be increased by forming the semiconductor film 504 which has a {110} plane. Then, in the case of forming a transistor as a semiconductor element, the bonding direction of the semiconductor film 504 is determined in consideration of a channel direction and crystal plane orientation.

Next, a surface of the semiconductor film 504 may be planarized by polishing. The planarization is not always necessary; however, the planarization makes it possible to improve the characteristics of an interface between semiconductor films 506 and 507 and a gate insulating film which are formed later. Specifically, the polishing may be chemical mechanical polishing (CMP), liquid jet polishing, or the like. The thickness of the semiconductor film 504 is reduced by the above-described planarization. The planarization may be performed on the semiconductor film 504 before being etched; alternatively, the planarization may be performed on the semiconductor films 506 and 507 formed by etching.

Not the polishing but etching may be performed on the surface of the semiconductor film 504 in order to planarize the surface of the semiconductor film 504. The etching may be performed by a dry etching method such as a reactive ion etching (RIE) method; for example, an inductively coupled plasma (ICP) etching method, an electron cyclotron resonance (ECR) etching method, a parallel plate (capacitive coupled type) etching method, a magnetron plasma etching method, a dual-frequency plasma etching method, a helicon wave plasma etching method, or the like may be used.

For example, when an ICP etching method is used, etching may be performed under the condition where the flow rate of chlorine as an etching gas is 40 sccm to 100 sccm, the electric power applied to a coil type electrode is 100 W to 200 W, the electric power applied to a lower electrode (on the bias side) is 40 W to 100 W, and the reaction pressure is 0.5 Pa to 1.0 Pa. For example, the thickness of the semiconductor film 504 can be reduced to about 50 nm to 60 nm by performing the etching under the condition where the flow rate of chlorine as an etching gas is 100 sccm, the reaction pressure is 1.0 Pa, the temperature of the lower electrode is 70° C., the RF (13.56 MHz) electric power applied to the coil-shaped electrode is 150 W, the electric power applied to the lower electrode (on the bias side) is 40 W, and the etching time is about 25 seconds to 27 seconds. For the etching gas, a chlorine-based gas such as chlorine, boron chloride, silicon chloride, or carbon tetrachloride; a fluorine-based gas such as carbon tetrafluoride, sulfur fluoride, or nitrogen fluoride; or oxygen can be used as appropriate.

The etching can not only make the semiconductor film 504 to have a film thickness optimum for a semiconductor element that is to be formed later but also planarize the surface of the semiconductor film 504.

Note that in the semiconductor film 504 closely in contact with the base substrate 503, crystal defects are formed owing to the formation of the embrittlement layer 502 and the split along the embrittlement layer 502, or planarity of the surface of the semiconductor film 504 is impaired. Thus, in one embodiment of the present invention, in order to reduce crystal defects and improve planarity, the semiconductor film 504 is irradiated with a laser beam after a process of removing an oxide film such as a natural oxide film which is formed on the surface of the semiconductor film 504.

In this embodiment of the present invention, the semiconductor film 504 is immersed in DHF having a hydrogen fluoride concentration of 0.5 wt % for 110 seconds, whereby the oxide film is removed.

The laser beam irradiation is preferably performed with such an energy density that the semiconductor film 504 is partly melted. This is because when the semiconductor film 504 is completely melted, generation of microcrystals due to recrystallization of the semiconductor film 504 is accompanied with disordered nucleation of the semiconductor film 504 in a liquid phase and crystallinity of the semiconductor film 504 is lowered. By partly melting the semiconductor film 504, so-called longitudinal growth in which crystal growth proceeds from an unmelted solid portion occurs in the semiconductor film 504. Due to the recrystallization by the longitudinal growth, crystal defects of the semiconductor film 504 are decreased and crystallinity thereof is recovered. The state in which the semiconductor film 504 is completely melted indicates the state in which the semiconductor film 504 is melted to be in a liquid phase to the interface with the insulating film 501. On the other hand, the state in which the semiconductor film 504 is partly melted indicates the state in which an upper part thereof is melted and is in a liquid phase and a lower part thereof is in a solid phase.

As this laser beam irradiation, pulsed laser beam irradiation is preferable for partly melting the semiconductor film 504. For example, in the case of a pulsed laser, the repetition rate is less than or equal to 1 MHz and the pulse width is greater than or equal to 10 nanoseconds and less than or equal to 500 nanoseconds. For example, a XeCl excimer laser having a repetition rate of 10 Hz to 300 Hz, a pulse width of 25 nanoseconds, and a wavelength of 308 nm can be used.

As the laser beam, a fundamental wave or a second harmonic of a solid-state laser, which is selectively absorbed by a semiconductor, is preferably used. Specifically, for example, a laser beam having a wavelength in the range of greater than or equal to 250 nm and less than or equal to 700 nm can be used. The energy of the laser beam can be determined in consideration of the wavelength of the laser beam, the skin depth of the laser beam, the thickness of the semiconductor film 504, or the like. For example, in the case where the thickness of the semiconductor film 504 is approximately 120 nm and a pulsed laser that emits laser beam having a wavelength of 308 nm is used, the energy density of the laser beam may be set to 600 mJ/cm² to 700 mJ/cm².

The following pulsed lasers can be used: an Ar laser, a Kr laser, an excimer laser, a CO₂ laser, a YAG laser, a Y₂O₃ laser, a YVO₄ laser, a YLF laser, a YAlO₃ laser, a glass laser, a ruby laser, an alexandrite laser, a Ti:sapphire laser, a copper vapor laser, and a gold vapor laser.

In this embodiment, in the case where the thickness of the semiconductor film 504 is approximately 146 nm, the laser beam irradiation can be performed in the following manner. As a laser emitting a laser beam, a XeCl excimer laser (wavelength: 308 nm, pulse width: 20 nanoseconds, and repetition rate: 30 Hz) is used. The cross section of the laser light is shaped into a linear form with a size of 0.4 mm×120 mm through an optical system. The semiconductor film 504 is irradiated with the laser beam with laser scanning speed of 0.5 mm/s. Then, through the laser beam irradiation, a semiconductor film 505 whose crystal defects have been repaired is formed as illustrated in FIG. 7E.

Note that the laser beam irradiation is preferably performed in an inert atmosphere such as a rare gas atmosphere or a nitrogen atmosphere, or a reduced-pressure atmosphere. In the case of the above atmosphere, the laser light irradiation may be performed in an airtight chamber whose atmosphere is controlled. When the chamber is not used, the laser beam irradiation in an inert atmosphere can be achieved by spraying an inert gas such as a nitrogen gas to the surface to be irradiated with the laser beam. The laser beam irradiation is performed in an inert atmosphere or a reduced-pressure atmosphere instead of an air atmosphere, whereby the natural oxide film is further prevented from being formed, cracks or pitch stripes can be prevented from being formed in the semiconductor film 505 which is formed after the laser beam irradiation, planarity of the semiconductor film 505 can be improved, and the applicable energy range for the laser beam can be widened.

The laser beam preferably has its cross section shaped into a linear form with homogenous energy distribution through an optical system. Accordingly, the laser beam irradiation can be performed homogenously at high throughput. With the beam length of the laser beam longer than one side of the base substrate 503, the entire semiconductor film 504 attached to the base substrate 503 can be irradiated with the laser beam by scanning the laser beam once. When the beam length of the laser beam is shorter than one side of the base substrate 503, the beam length may be set so that the entire semiconductor film 504 attached to the base substrate 503 can be irradiated with the laser light by scanning several times.

In order to perform the laser beam irradiation in a reduced-pressure atmosphere or an inert atmosphere such as a rare gas atmosphere or a nitrogen atmosphere, the laser beam irradiation may be performed in an airtight chamber whose atmosphere is controlled. When the chamber is not used, the laser beam irradiation in an inert atmosphere can be achieved by spraying an inert gas such as a nitrogen gas to the surface to be irradiated with the laser beam. The laser beam irradiation is performed in an inert atmosphere or a reduced-pressure atmosphere instead of an air atmosphere, whereby the natural oxide film is further prevented from being formed, cracks or pitch stripes can be prevented from being formed in the semiconductor film 505 which is formed after the laser beam irradiation, planarity of the semiconductor film 505 can be improved, and the applicable energy range for the laser beam can be widened.

In the case where the surface of the semiconductor film 504 is planarized by dry etching before the laser beam irradiation, damages such as crystal defects might be generated on and near the surface of the semiconductor film 504 owing to the dry etching. However, the above-described laser beam irradiation can recover even the damages caused by the dry etching.

Next, after the laser beam irradiation, the surface of the semiconductor film 505 may be etched. When the surface of the semiconductor film 505 is etched after the laser beam irradiation, the surface of the semiconductor film 504 is not necessarily etched before the laser beam irradiation. Moreover, when the surface of the semiconductor film 504 is etched before the laser beam irradiation, the surface of the semiconductor film 505 is not necessarily etched after the laser beam irradiation. Alternatively, the surface of the semiconductor film 505 may be etched after the laser beam irradiation and before the laser beam irradiation.

The etching can not only make the semiconductor film 505 to have a film thickness optimum for a semiconductor element that is to be formed later but also planarize the surface of the semiconductor film 505.

After the laser beam irradiation, the semiconductor film 505 is preferably subjected to heat treatment at higher than or equal to 500° C. and lower than or equal to 650° C. This heat treatment can eliminate defects of the semiconductor film 505 which have not been repaired by the laser beam irradiation and can reduce distortion of the semiconductor film 505. For this heat treatment, a rapid thermal annealing (RTA) apparatus, a resistive heating furnace, or a microwave heating apparatus can be used. For the RTA apparatus, a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. For example, when a resistance heating furnace is used, heat treatment may be performed at 600° C. for 4 hours.

Next, as illustrated in FIG. 8A, the semiconductor film 505 is partly etched to form the island-shaped semiconductor films 506 and 507. When the semiconductor film 505 is further etched, a region of an end portion of the semiconductor film 505 where the bonding strength is not sufficient can be removed. Although the semiconductor films 506 and 507 are formed by etching one semiconductor film 505 in this embodiment, the number of semiconductor films which are formed is not limited to two.

Note that the surface of the bond substrate 500 from which the semiconductor film 505 is separated is planarized, whereby a semiconductor film 505 can be separated again from the bond substrate 500.

Specifically, the insulating film 501 which remains mainly at edge portions of the bond substrate 500 is removed by etching or the like. In the case where the insulating film 501 is formed using silicon oxide, silicon oxynitride, or silicon nitride oxide, wet etching using hydrofluoric acid can be employed.

Next, projections formed at the edge portions of the bond substrate 500 owing to the separation of the semiconductor film 505 and the remaining embrittlement layer which contains hydrogen excessively are removed. For the etching of the bond substrate 500, wet etching is preferably used, and a tetramethylammonium hydroxide (abbreviation: TMAH) solution can be used as an etchant.

Then, the surface of the bond substrate 500 is polished. For the polishing, CMP can be used. To smooth the surface of the bond substrate 500, the surface is desirably polished by approximately 1 μm to 10 μm in thickness. After the polishing, RCA cleaning using hydrofluoric acid or the like is performed because abrasive particles and the like are left on the surface of the bond substrate 500.

By reusing the bond substrate 500, the cost of a material of the semiconductor substrate can be reduced.

In order to control threshold voltage, an impurity element imparting p-type conductivity such as boron, aluminum, or gallium or an impurity element imparting n-type conductivity such as phosphorus or arsenic may be added to the semiconductor films 506 and 507. The impurity element for controlling threshold voltage may be added to the semiconductor film before being patterned or on the semiconductor films 506 and 507 formed after the patterning. Alternatively, the impurity element for controlling the threshold voltage may be added to a bond substrate. Further alternatively, the impurity element may be added to the bond substrate in order to roughly control the threshold voltage, and the impurity element may be further added to the semiconductor film before being patterned or the semiconductor films 506 and 507 which are formed after the patterning in order to finely control the threshold voltage.

Next, gate insulating films 508 are formed to cover the semiconductor films 506 and 507, as illustrated in FIG. 8B. The gate insulating films 508 can be formed by oxidation or nitridation of surfaces of the semiconductor films 506 and 507 by high-density plasma treatment. The high-density plasma treatment is performed, for example, by using a mixed gas of an inert gas such as He, Ar, Kr, or Xe, and oxygen, nitrogen oxide, ammonia, nitrogen, hydrogen, or the like. In this case, by performing excitation of plasma with introduction of a microwave, plasma with a low electron temperature and high density can be generated. The surfaces of the semiconductor films are oxidized or nitrided by oxygen radicals (which include OH radicals in some cases) or nitrogen radicals (which include NH radicals in some cases) produced by such high-density plasma, whereby an insulating film of 1 nm to 20 nm thick, desirably 5 nm to 10 nm thick, is formed in contact with the semiconductor films. This insulating film of 5 nm to 10 nm thick is used as the gate insulating film 508. For example, dinitrogen monoxide (N₂O) is diluted with Ar by 1 to 3 times (flow rate) and a microwave (2.45 GHz) electric power of 3 kW to 5 kW is applied with a pressure of 10 Pa to 30 Pa to oxidize or nitride the surfaces of the semiconductor films 506 and 507. By this treatment, an insulating film having a thickness of 1 nm to 10 nm (preferably 2 nm to 6 nm) is formed. Further, nitrous oxide (N₂O) and silane (SiH₄) are introduced and a microwave (2.45 GHz) electric power of 3 kW to 5 kW is applied to the insulating film at a pressure of 10 Pa to 30 Pa to form a silicon oxynitride film by a vapor-phase growth method, which is to be the gate insulating film. With a combination of a solid-phase reaction and a reaction by a vapor deposition method, the gate insulating film with low interface state density and excellent withstand voltage can be formed.

Since the oxidation or nitridation of the semiconductor films by the high-density plasma treatment proceeds by a solid-phase reaction, the interface state density between the gate insulating film 508 and each of the semiconductor films 506 and 507 can be drastically decreased. Further, since the semiconductor films 506 and 507 are directly oxidized or nitrided by the high-density plasma treatment, variation in thickness of the insulating film to be formed can be suppressed. Moreover, in the case where the semiconductor film has crystallinity, the surface of the semiconductor film is oxidized with solid reaction by the high-density plasma treatment to restrain fast oxidation only in a crystal grain boundary; therefore, the gate insulating film with uniformity and low interface state density can be formed. A transistor, in which the insulating film formed by the high-density plasma treatment is included in a part of or the entire gate insulating film, may reduce variations in characteristics.

Alternatively, the gate insulating film 508 may be formed by thermally oxidizing the semiconductor films 506 and 507. Further alternatively, the gate insulating film 508 may be formed by a plasma CVD method, a sputtering method, or the like as a single layer or a stack of layers of a film containing one or more of silicon oxide, silicon nitride oxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminum oxide, and tantalum oxide.

Then, after forming a conductive film over the gate insulating films 508 as illustrated in FIG. 8C, the conductive film is processed (patterned) into a predetermined shape, so that an electrode 509 is formed over each of the semiconductor films 506 and 507. A CVD method, a sputtering method, or the like can be used for forming the conductive film. As the conductive film, tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), or the like can be used. Moreover, an alloy containing the above-mentioned metal as the main component or a compound containing the above-mentioned metal may be used. Alternatively, the conductive film may be formed of a semiconductor such as polycrystalline silicon doped with an impurity element such as phosphorus which imparts conductivity to the semiconductor film.

In the case of forming a two-layer conductive film, a first layer can be formed of tantalum nitride or tantalum and a second layer can be formed of tungsten. Moreover, the following combinations are given: tungsten nitride and tungsten, molybdenum nitride and molybdenum, aluminum and tantalum, aluminum and titanium, and the like. Since tungsten and tantalum nitride have high heat resistance, heat treatment for thermal activation can be performed after the two-layer conductive film is formed. Alternatively, as the combination of the two-layer conductive film, silicon doped with an impurity element imparting n-type conductivity and nickel silicide, silicon doped with an impurity element imparting n-type conductivity and tungsten silicide, or the like can be used.

Note that, although the electrode 509 is formed of a single-layer conductive film in this embodiment, this embodiment is not limited to this structure. The electrode 509 may be formed of a plurality of conductive films which is stacked. In the case of using a three-layer structure which is stacked with more than three conductive films, a stacked structure of a molybdenum film, an aluminum film, and a molybdenum film is preferable.

Note that the electrode 509 may be selectively formed by a droplet discharge method without using a mask.

Note that a droplet-discharge method means a method in which a predetermined pattern is formed by discharging or ejecting droplets containing a predetermined composition from fine pores, and an inkjet method is included in the category.

After the conductive film is formed, the electrode 509 can be etched into a desired tapered shape by using an ICP (inductively coupled plasma) etching method and appropriately controlling the etching condition (e.g., the amount of electric power applied to a coiled electrode layer, the amount of electric power applied to an electrode layer on the substrate side, or the electrode temperature on the substrate side). In addition, angles and the like of the tapered shapes may also be controlled by a shape of a mask. Note that as an etching gas, a chlorine-based gas such as chlorine, boron chloride, silicon chloride, or carbon tetrachloride; a fluorine-based gas such as carbon tetrafluoride, sulfur fluoride, or nitrogen fluoride; or oxygen can be used as appropriate.

Next, as shown in FIG. 8D, impurity elements imparting one conductivity type is added to the semiconductor films 506 and 507 by using the electrodes 509 as masks. In this embodiment, an impurity element imparting n-type conductivity (e.g., phosphorus or arsenic) is added to the semiconductor film 506, and an impurity element imparting p-type conductivity (e.g., boron) is added to the semiconductor film 507. Note that when the impurity element imparting p-type conductivity is added to the semiconductor film 507, the semiconductor film 506 to which the impurity element imparting n-type conductivity is to be added is covered with a mask or the like so that the impurity element imparting p-type conductivity is added selectively. On the other hand, when the impurity element imparting n-type conductivity is added to the semiconductor film 506, the semiconductor film 507 to which the impurity element imparting p-type conductivity is to be added is covered with a mask or the like so that the impurity element imparting n-type conductivity is added selectively. Alternatively, after adding an impurity element imparting one of p-type and n-type conductivity to the semiconductor films 506 and 507, an impurity element imparting the other conductivity may be added to only one of the semiconductor films 506 and 507 selectively at higher concentration than the previously added impurity. By the addition of the impurity elements, impurity regions 510 are formed in the semiconductor film 506, and impurity regions 511 are formed in the semiconductor film 507.

Next, as illustrated in FIG. 9A, sidewalls 512 are formed on side surfaces of the electrodes 509. For example, the sidewalls 512 can be formed in such a manner that an insulating film is newly formed so as to cover the gate insulating films 508 and the electrodes 509, and the insulating film is partly etched by anisotropic etching in which etching is performed mainly in a perpendicular direction. By the anisotropic etching, the newly formed insulating film is partly etched to form the sidewalls 512 at the side surfaces of the electrodes 509. Note that the gate insulating films 508 may also be partially etched by the anisotropic etching. The insulating film for forming the sidewalls 512 may be a single layer or a stack of layers of one or more of a silicon film, a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, and a film containing an organic material such as an organic resin formed by a LPCVD method, a plasma CVD method, a sputtering method, or the like. In this embodiment, a 100-nm-thick silicon oxide film is formed by a plasma CVD method. As the etching gas, a mixed gas of CHF₃ and helium can be used. Note that the process for forming the sidewalls 512 is not limited to this process.

Next, as shown in FIG. 9B, impurity elements imparting one conductivity type are added to the semiconductor films 506 and 507 with the electrodes 509 and the sidewalls 512 used as masks. Note that the impurity elements imparting the same conductivity type as the impurity elements which have been added in the previous step are added to the semiconductor films 506 and 507 at a higher concentration than in the previous step. Note that when the impurity element imparting p-type conductivity is added to the semiconductor film 507, the semiconductor film 506 to which the impurity element imparting n-type conductivity is to be added is covered with a mask or the like so that the impurity element imparting p-type conductivity is added selectively. In contrast, when the impurity element imparting n-type conductivity is added to the semiconductor film 506, the semiconductor film 507 to which the impurity element imparting p-type conductivity is to be added is covered with a mask or the like so that the impurity element imparting n-type conductivity is added selectively.

In addition, by the above-described addition of the impurity elements, a pair of high-concentration impurity regions 513, a pair of low-concentration impurity regions 514, and a channel formation region 515 are formed in the semiconductor film 506. Moreover, by the addition of the impurity elements, a pair of high-concentration impurity regions 516, a pair of low-concentration impurity regions 517, and a channel formation region 518 are formed in the semiconductor film 507. The high-concentration impurity regions 513 and 516 serve as a source region or a drain region, and the low-concentration impurity regions 514 and 517 serve as LDD (lightly doped drain) regions. Note that the LDD regions are not necessarily provided, and only impurity regions that serve as source and drain regions may be formed. Alternatively, the LDD region may be formed on either the source region side or the drain region side.

Note that in the case of a transistor using silicon, a source region and a drain region respectively serve as a source electrode and a drain electrode.

Note that the sidewalls 512 formed over the semiconductor film 507 and the sidewalls 512 formed over the semiconductor film 506 may be formed to have the same width or different widths in the carrier flow direction. It is preferable that the width of each sidewall 512 over the semiconductor film 507 which constitutes a part of a p-channel transistor be larger than the width of each sidewall 512 over the semiconductor film 506 which constitutes a part of an n-channel transistor. This is because boron which is added for forming a source region and a drain region in the p-channel transistor is easily diffused and a short channel effect is easily induced. When the width of each sidewall 512 in the p-channel transistor is made larger than that of each sidewall 512 in the n-channel transistor, boron can be added to the source region and the drain region at high concentration, and thus the resistance of the source region and the drain region can be reduced.

Next, in order to further reduce the resistance of the source region and the drain region, silicide is formed in the semiconductor films 506 and 507, so that silicide layers may be formed. The silicide is formed in such a manner that a metal is brought into contact with the semiconductor films, and silicon in the semiconductor films is made to react with the metal by heat treatment, a GRTA method, an LRTA method, or the like. The silicide layer may be formed of cobalt silicide or nickel silicide. In the case where the semiconductor films 506 and 507 are thin, silicide reaction may proceed to bottoms of the semiconductor films 506 and 507 in this region. As a metal material used for the formation of the silicide, the following can be used: titanium (Ti), nickel (Ni), tungsten (W), molybdenum (Mo), cobalt (Co), zirconium (Zr), hafnium (Hf), tantalum (Ta), vanadium (V), neodymium (Nd), chromium (Cr), platinum (Pt), palladium (Pd), or the like. Alternatively, the silicide may be formed by laser irradiation, light irradiation using a lamp, or the like.

Through the above-described steps, an n-channel transistor 520 and a p-channel transistor 521 are formed.

After the step illustrated in FIG. 9B is completed, a transistor including an oxide semiconductor is manufactured over the n-channel transistor 520 and the p-channel transistor 521.

First, as shown in FIG. 10A, an insulating film 530 is formed to cover the n-channel transistor 520 and the p-channel transistor 521. By providing the insulating film 530, oxidation of surfaces of the electrodes 509 can be prevented when heat treatment is performed. Specifically, it is preferable to use silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum nitride, aluminum oxide, silicon oxide, or the like for the insulating film 530. In this embodiment, a silicon oxynitride film having a thickness of approximately 50 nm is used as the insulating film 530.

Next, as shown in FIG. 10B, an insulating film 531 and an insulating film 532 are formed over the insulating film 530 to cover the n-channel transistor 520 and the p-channel transistor 521. The insulating films 531 and 532 are formed using materials which can withstand a temperature of heat treatment in a later manufacturing step. Specifically, an inorganic insulating film of silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum nitride, aluminum nitride oxide, or the like can be used for the insulating films 531 and 532, for example.

Note that the insulating films 531 and 532 are stacked over the insulating film 530 in this embodiment; however, the insulating film formed over the insulating film 530 may be an insulating film of a single layer or an insulating layer in which three or more layers are stacked.

A surface of the insulating film 532 may be planarized by a CMP method or the like.

Next, as illustrated in FIG. 10B, a gate electrode 601 and an electrode 602 are formed over the insulating film 532.

The gate electrode 601 and the electrode 602 can be formed with a single layer or a stacked layer using one or more conductive films using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, neodymium, or scandium; an alloy material which includes any of these metal materials as a main component; or a nitride of any of these metals. Note that aluminum or copper can also be used as such metal materials as long as aluminum or copper can withstand a temperature of heat treatment performed in a later process. Aluminum or copper is preferably combined with a refractory metal material so as to prevent a heat resistance problem and a corrosive problem. As the refractory metal material, molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium, or the like can be used.

For example, as a two-layer structure of the gate electrode 601 and the electrode 602, the following structures are preferable: a two-layer structure in which a molybdenum film is stacked over an aluminum film, a two-layer structure in which a molybdenum film is stacked over a copper film, a two-layer structure in which a titanium nitride film or a tantalum nitride film is stacked over a copper film, and a two-layer structure in which a titanium nitride film and a molybdenum film are stacked. As a three-layer structure of the gate electrode 601 and the electrode 602, the following structure is preferable: a stacked structure containing an aluminum film, an alloy film of aluminum and silicon, an alloy film of aluminum and titanium, or an alloy film of aluminum and neodymium in a middle layer and any of a tungsten film, a tungsten nitride film, a titanium nitride film, and a titanium film in a top layer and a bottom layer.

Further, a light-transmitting oxide conductive film of indium oxide, indium tin oxide, an indium oxide-zinc oxide alloy, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, or the like may be used as the gate electrode 601 and the electrode 602.

The thicknesses of the gate electrode 601 and the electrode 602 are 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, a conductive film for the gate electrode is formed to a thickness of 150 nm by a sputtering method using a tungsten target, and then the conductive film is processed (patterned) into a desired shape by etching; thus, the gate electrode 601 and the electrode 602 are formed. Note that end portions of the formed gate electrode are preferably tapered, in which case coverage with a gate insulating film stacked thereover is improved. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

Next, as illustrated in FIG. 10C, a gate insulating film 603 is formed over the gate electrode 601 and the electrode 602. The gate insulating film 603 is formed using a film having a single layer or a stacked layer of one or more of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, a hafnium oxide film, and a tantalum oxide film by a plasma CVD method, a sputtering method, or the like. It is preferable that the gate insulating film 603 include impurities such as moisture or hydrogen as little as possible. In the case of forming a silicon oxide film by a sputtering method, a silicon target or a quartz target is used as a target, and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas.

Here, the oxide semiconductor that is made to be an intrinsic semiconductor or a substantially intrinsic semiconductor (the oxide semiconductor that is highly purified) by removal of impurities is extremely sensitive to an interface state and an interface electric charge; thus, an interface between the highly-purified oxide semiconductor and the gate insulating film 603 is important. Therefore, the gate insulating film (GI) that is in contact with the highly-purified oxide semiconductor needs to have a higher quality.

For example, high-density plasma CVD using microwaves (2.45 GHz) is preferable because a dense high-quality insulating film having high withstand voltage can be formed. This is because when the highly-purified oxide semiconductor is closely in contact with the high-quality gate insulating film, the interface state can be reduced and interface properties can be favorable.

Needless to say, a different deposition method such as a sputtering method or a plasma CVD method can be used as long as a high-quality insulating film can be formed as a gate insulating film. Alternatively, an insulating film whose film quality and interface characteristics between the gate insulating film and the oxide semiconductor are improved by heat treatment performed after formation of the insulating film may be used. In any case, any insulating film can be used as long as film quality as a gate insulating film is high, interface state density between the gate insulating film and an oxide semiconductor is decreased, and a favorable interface can be formed.

The gate insulating film 603 may have a structure in which an insulating film formed using a material having a high barrier property and an insulating film formed using a silicon oxide film, a silicon oxynitride film, or the like having a lower proportion of nitrogen are stacked. In that case, the insulating film such as a silicon oxide film or a silicon oxynitride film is formed between the insulating film having a high barrier property and the oxide semiconductor film. As the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be given, for example. The insulating film having a high barrier property is used, so that impurities in an atmosphere, such as moisture or hydrogen, or impurities included in the substrate, such as an alkali metal or a heavy metal, can be prevented from entering the oxide semiconductor film, the gate insulating film 603, or the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. In addition, the insulating film having a lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film is formed so as to be in contact with the oxide semiconductor film, so that the insulating film having a high barrier property can be prevented from being in direct contact with the oxide semiconductor film.

For example, a stacked-layer film with a thickness of 100 nm may be formed as the gate insulating film 603 as follows: a silicon nitride film (SiN_(y) (y>0)) with a thickness of greater than or equal to 50 nm and less than or equal to 200 nm is formed by a sputtering method as a first gate insulating film, and a silicon oxide film (SiO_(x) (x>0)) with a thickness of greater than or equal to 5 nm and less than or equal to 300 nm is stacked over the first gate insulating film as a second gate insulating film. The thickness of the gate insulating film 603 may be set as appropriate depending on the desired characteristics of the transistor. The thickness may be approximately 350 nm to 400 nm.

In this embodiment, the gate insulating film 603 is formed to have a structure in which a 100-nm-thick silicon oxide film formed by a sputtering method is stacked over a 50-nm-thick silicon nitride film formed by a sputtering method.

Note that in order that hydrogen, hydroxyl group, and moisture are contained as little as possible in the gate insulating film 603, it is preferable that the base substrate 503 over which the gate electrode 601 and the electrode 602 are formed be preheated in a preheating chamber of the sputtering apparatus, so that impurities such as moisture or hydrogen adsorbed onto the base substrate 503 is eliminated and evacuated, as pretreatment before film formation. The temperature for the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an evacuation unit provided in the preheating chamber, a cryopump is preferable. Note that this preheating treatment can be omitted.

Next, over the gate insulating film 603, an oxide semiconductor film having a thickness of greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm, more preferably greater than or equal to 3 nm and less than or equal to 20 nm is formed. The oxide semiconductor film is formed by a sputtering method using an oxide semiconductor as a target. Moreover, the oxide semiconductor film can be formed by a sputtering method under a rare gas (for example, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere including a rare gas (for example, argon) and oxygen.

Note that before the oxide semiconductor film is formed by a sputtering method, dust attached to a surface of the gate insulating film 603 is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering means a method in which a surface of a substrate is modified by application of voltage to the substrate side, not to a target side, in an argon atmosphere with the use of an RF power source and by generating plasma in the vicinity of the substrate. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used.

The above oxide semiconductor can be used for the oxide semiconductor film.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-based non-single-crystal film with a thickness of 30 nm, which is obtained by a sputtering method using a metal oxide target including indium (In), gallium (Ga), and zinc (Zn), is used. As the target, for example, a metal oxide target having a composition ratio of metals, In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. The target may contain SiO₂ at greater than or equal to 2 wt % and less than or equal to 10 wt %. The fill rate of the metal oxide target including In, Ga, and Zn is higher than or equal to 90% and lower than or equal to 100%, preferably higher than or equal to 95% and lower than or equal to 99.9%. With use of a metal oxide target with high fill rate, the deposited oxide semiconductor film has high density.

In this embodiment, the oxide semiconductor film is formed over the base substrate 503 as follows: the substrate is held in a treatment chamber with pressure reduced, moisture remaining in the treatment chamber is being removed, a sputtering gas from which hydrogen and moisture are removed is introduced, and the above-described target is used. At that time, the substrate may be heated at higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Film formation is performed while the substrate is heated, whereby the impurity concentration in the oxide semiconductor film formed can be reduced. In addition, damage by sputtering can be reduced. In order to remove moisture remaining in the treatment chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo pump provided with a cold trap. In the treatment chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), (more preferably, also a compound containing a carbon atom), and the like are removed, whereby the impurity concentration in the oxide semiconductor film formed in the treatment chamber can be reduced.

As one example of the deposition condition, the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct current (DC) power source is 0.5 kW, and the atmosphere is an oxygen atmosphere (the proportion of the oxygen flow rate is 100%). Note that a pulse direct current (DC) power source is preferable because powder substances called particles generated in film formation can be reduced and the film thickness can be uniform.

Note that in order that hydrogen, hydroxyl group, and moisture are contained as little as possible in the oxide semiconductor film, it is preferable that the base substrate 503 over which layers up to and including the gate insulating film 603 are formed be preheated in a preheating chamber of the sputtering apparatus, so that impurities such as hydrogen or moisture adsorbed onto the base substrate 503 is eliminated and evacuated, as pretreatment before film formation. The temperature for the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an evacuation unit provided in the preheating chamber, a cryopump is preferable. Note that this preheating treatment can be omitted. Further, this preheating may be similarly performed on the base substrate 503 over which layers up to and including a source electrode 607 and a drain electrode 608, wirings 609 to 611 are formed, before the formation of an insulating film 612.

Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power supply, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case where an insulating film is formed, and a DC sputtering method is mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber.

Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge can be used.

Further, as a deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, or a bias sputtering method in which voltage is also applied to a substrate during deposition can be used.

The gate insulating film 603 and the oxide semiconductor film may be formed successively without exposure to air. Successive film formation without exposure to air makes it possible to obtain each interface between stacked layers, which is not contaminated by atmospheric components or impurity elements floating in air, such as water, hydrocarbon, or the like. Therefore, variation in characteristics of the transistor can be reduced.

Next, as illustrated in FIG. 10C, the oxide semiconductor film is processed (patterned) into a desired shape by etching or the like, whereby an island-shaped oxide semiconductor film 605 is formed over the gate insulating film 603 in a position where the island-shaped oxide semiconductor film 605 overlaps with the gate electrode 601.

A resist mask for forming the island-shaped oxide semiconductor film 605 may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

Note that the etching for forming the island-shaped oxide semiconductor film 605 may be dry etching, wet etching, or both dry etching and wet etching. As the etching gas for dry etching, a gas containing chlorine (chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃), silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferably used. Alternatively, a gas containing fluorine (fluorine-based gas such as carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching) method or an ICP (inductively coupled plasma) etching method can be used. In order to etch the film into a desired shape, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate.

As an etchant used for wet etching, a solution obtained by mixing phosphoric acid, acetic acid, and nitric acid, an ammonia peroxide mixture (hydrogen peroxide water at 31 wt %: ammonia water at 28 wt %: water=5:2:2), or the like can be used. Alternatively, ITO-07N (manufactured by Kanto Chemical Co., Inc.) may be used. The etchant after the wet etching is removed together with the etched materials by cleaning. The waste liquid including the etchant and the material etched off may be purified and the material may be reused. When a material such as indium contained in the oxide semiconductor film is collected from the waste liquid after the etching and reused, the resources can be efficiently used and the cost can be reduced.

Note that it is preferable that reverse sputtering be performed before the formation of a conductive film in a subsequent step so that a resist residue or the like that attaches onto surfaces of the island-shaped oxide semiconductor film 605 and the gate insulating film 603 is removed.

Next, the oxide semiconductor film 605 is subjected to heat treatment under a reduced-pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen atmosphere, or an ultra-dry air (the moisture amount is 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, more preferably 10 ppb or less, in the case where measurement is performed by a dew point meter in a cavity ring down laser spectroscopy (CRDS) method). By performing the heat treatment on the oxide semiconductor film 605, moisture or hydrogen in the oxide semiconductor film 605 can be eliminated. Specifically, the heat treatment may be performed at higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to the strain point of the glass substrate), preferably higher than or equal to 550° C. and lower than or equal to 750° C. For example, the heat treatment may be performed at 600° C. for greater than or equal to 3 minutes and less than or equal to 6 minutes. With an RTA method for the heat treatment, dehydration or dehydrogenation can be performed in a short time; therefore, treatment can be performed even at a temperature higher than the strain point of a glass substrate. Alternatively, the heat treatment may be performed in the state where the substrate temperature is 450° C. for approximately 1 hour.

In this embodiment, heat treatment is performed on the oxide semiconductor film 605 at a substrate temperature of 600° C. for 6 minutes in a nitrogen atmosphere with the use of an electrical furnace that is one of heat treatment apparatuses, and then, the oxide semiconductor film is not exposed to air and water or hydrogen is prevented from entering the oxide semiconductor film.

Note that a heat treatment apparatus is not limited to an electrical furnace, and a heat treatment apparatus may be provided with a device for heating an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon is used.

For example, the heat treatment can employ GRTA in which the substrate is moved into an inert gas heated at a high temperature of 650° C. to 700° C., and heated for several minutes there, and then the substrate is moved out of the high-temperature inert gas. With GRTA, high-temperature heat treatment for a short period of time can be achieved.

Note that in the heat treatment, it is preferable that moisture, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. It is preferable that the purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower).

When impurities such as moisture or hydrogen is added to an oxide semiconductor, in a gate bias-temperature stress test (BT test, the test condition is, for example, at 85° C. with 2×10⁶ V/cm for 12 hours), a bond between the impurities and a main component of the oxide semiconductor is broken by a high electrical field (B: bias) and high temperature (T: temperature), and a dangling bond generated causes drift of the threshold voltage (Vth). However, in the above-described manner, by improving the interface characteristics between the gate insulating film and the oxide semiconductor film and removing impurities, particularly hydrogen, moisture, or the like, in the oxide semiconductor film as much as possible, a transistor which remains stable even with respect to the BT test can be obtained.

Through the above-described steps, the concentration of hydrogen in the oxide semiconductor film 605 can be reduced and the island-shaped oxide semiconductor film is highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature of lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that productivity can be increased. In addition, by using the oxide semiconductor film in which the hydrogen concentration is reduced and purity is improved, it is possible to manufacture a transistor with high withstand voltage, a reduced short-channel effect, and a high on-off ratio.

Note that in the case where the oxide semiconductor film is heated, although depending on a material of the oxide semiconductor film or heating conditions, plate-shaped crystals are formed over the top surface of the oxide semiconductor film in some cases. The plate-shaped crystals preferably form a single crystal body in which crystals are c-axis-oriented in a direction generally perpendicular to the surface of the oxide semiconductor film. Even when the plate-shaped crystals do not form a single crystal body, the plate-shaped crystals preferably form a polycrystal body in which each crystal is c-axis-oriented in a direction generally perpendicular to the surface of the oxide semiconductor film. In the above-described polycrystal body, in addition to the c-axis orientation, the crystals preferably have identical a-b planes, a-axes, or b-axes. Note that in the case where a surface of the base of the oxide semiconductor film is uneven, the plate-shaped crystals form a polycrystal body. Therefore, it is preferable that the surface of the base is as planar as possible.

Next, the insulating film 530, the insulating film 531, the insulating film 532, and the gate insulating film 603 are partly etched, so that contact holes reaching the high-concentration impurity regions 513 included in the n-channel transistor 520, contact holes reaching the high-concentration impurity regions 516 included in the p-channel transistor 521, and a contact hole reaching the electrode 602 are formed. Then, a conductive film used for the source electrode or the drain electrode (including a wiring formed in the same layer as the source electrode or the drain electrode) is formed over the oxide semiconductor film 605 by a sputtering method or a vacuum evaporation method, and then the conductive film is patterned by etching or the like, whereby as illustrated in FIG. 11A, the source electrode 607 and the drain electrode 608 over the oxide semiconductor film 605, a wiring 609 in contact with the electrode 602 and the high-concentration impurity region 513, a wiring 610 in contact with the high-concentration impurity region 516, and a wiring 611 in contact with the high-concentration impurity region 513 and the high-concentration impurity region 516 are formed.

As the material of the conductive film to be the source electrode and the drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode), there are an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy including any of the above elements as a component; an alloy film any of these elements in combination; and the like. Further, a structure may be employed in which a film formed using a high-melting point metal such as Cr, Ta, Ti, Mo, or W is stacked over either the lower side or the upper side of a metal film such as Al or Cu. Still alternatively, when an Al material to which an element preventing generation of hillocks and whiskers in an Al film, such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Y, is added is used, heat resistance can be increased.

Further, the conductive film may have a single-layer structure or a layered structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given.

Alternatively, the conductive film to be the source and drain electrodes (including a wiring formed in the same layer as the source and drain electrodes) may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium tin oxide (In₂O₃—SnO₂, abbreviated to ITO), an alloy of indium oxide and zinc oxide (In₂O₃—ZnO), or any of the metal oxide materials containing silicon or silicon oxide can be used.

In the case where heat treatment is performed after formation of the conductive film, the conductive film preferably has heat resistance enough to withstand the heat treatment.

Note that each material and etching conditions are adjusted as appropriate so that the oxide semiconductor film 605 is not removed in etching of the conductive film as much as possible. Depending on the etching conditions, an exposed portion of the island-shaped oxide semiconductor film 605 is partly etched, so that an island-shaped oxide semiconductor film 605 having a groove (a recessed portion) is formed in some cases.

In this embodiment, since the titanium film is used as the conductive film, wet etching can be selectively performed on the conductive film with the use of an ammonia peroxide mixture (hydrogen peroxide water at 31 wt %: ammonia water at 28 wt %: water=5:2:2), in which case part of the oxide semiconductor film 605 is also etched in some cases. Alternatively, dry etching may be performed on the conductive film with the use of a gas containing chlorine (Cl₂), boron chloride (BCl₃), or the like.

In order to reduce the number of photomasks and steps in a photolithography process, etching may be performed with the use of a resist mask formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. A resist mask formed with the use of a multi-tone mask has a plurality of thicknesses and further can be changed in shape by etching; therefore, the resist mask can be used in a plurality of etching steps for processing into different patterns. Therefore, a resist mask corresponding to at least two kinds or more of different patterns can be formed by one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of corresponding photolithography process can be also reduced, whereby simplification of a process can be realized.

Next, plasma treatment is performed using a gas such as N₂O, N₂, or Ar. By the plasma treatment, water or the like which attaches or is adsorbed to an exposed surface of the oxide semiconductor film is removed. Plasma treatment may be performed using a mixture gas of oxygen and argon as well.

After the plasma treatment, as illustrated in FIG. 11B, the insulating film 612 is formed so as to cover the source electrode 607, the drain electrode 608, the wirings 609 to 611, and the oxide semiconductor film 605. The insulating film 612 preferably contains impurities such as moisture or hydrogen as little as possible, and the insulating film 612 may be formed using a single-layer insulating film or a plurality of insulating films stacked. When hydrogen is contained in the insulating film 612, entry of the hydrogen to the oxide semiconductor film, or extraction of oxygen in the oxide semiconductor film by hydrogen may occur, thereby causing a backchannel portion of the oxide semiconductor film to have lower resistance (to be n-type), so that a parasitic channel may be formed. Therefore, it is preferable that a formation method in which hydrogen is not used is employed in order to form the insulating film 612 containing hydrogen as little as possible. A material having a high barrier property is preferably used for the insulating film 612. For example, as the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be used. When a plurality of insulating films stacked is used, an insulating film having a lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film is formed on the side closer to the oxide semiconductor film 605 than the insulating film having a high barrier property. Then, the insulating film having a high barrier property is formed so as to overlap with the source electrode 607, the drain electrode 608, and the oxide semiconductor film 605 with the insulating film having a lower proportion of nitrogen between the insulating film having a high barrier property and the source electrode 607, the drain electrode 608, and the island-shaped oxide semiconductor film 605. When the insulating film having a high barrier property is used, impurities such as moisture or hydrogen can be prevented from entering the oxide semiconductor film 605, the gate insulating film 603, or the interface between the oxide semiconductor film 605 and another insulating film and the vicinity thereof. In addition, the insulating film having a lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film is formed so as to be in contact with the oxide semiconductor film 605, so that the insulating film formed using a material having a high barrier property can be prevented from being in direct contact with the oxide semiconductor film 605.

In this embodiment, the insulating film 612 having a structure in which a silicon nitride film having a thickness of 100 nm formed with a sputtering method is stacked over a silicon oxide film having a thickness of 200 nm formed with a sputtering method is formed. The substrate temperature in film formation may be higher than or equal to room temperature and lower than or equal to 300° C. and in this embodiment, is 100° C.

Note that after the insulating film 612 is formed, heat treatment may be performed. The heat treatment is preferably performed in a reduced-pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or an ultra dry air atmosphere (the moisture amount is 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, more preferably 10 ppb or less, in the case where measurement is performed by a dew point meter in a cavity ring down laser spectroscopy (CRDS) method) at higher than or equal to 200° C. and lower than or equal to 400° C., for example, higher than or equal to 250° C. and lower than or equal to 350° C. In this embodiment, heat treatment for 1 hour at 250° C. in a nitrogen atmosphere is performed. Alternatively, RTA treatment for a short time at a high temperature may be performed before the formation of the source electrode 607, the drain electrode 608, and the wirings 609 to 611 in a manner similar to the heat treatment performed on the oxide semiconductor film. Even when oxygen deficiency is generated in the oxide semiconductor film 605 owing to the pervious heat treatment on the oxide semiconductor film, by performing heat treatment after the insulating film 612 containing oxygen is formed in contact with the exposed region of the oxide semiconductor film 605 provided between the source electrode 607 and the drain electrode 608, oxygen is supplied to the oxide semiconductor film 605. By supplying oxygen to the region of the oxide semiconductor film 605 in contact with the insulating film 612, oxygen deficiency that serves as a donor is reduced and the stoichiometric composition ratio can be satisfied. As a result, the oxide semiconductor film 605 can be made to be an i-type semiconductor film or a substantially i-type semiconductor film. Accordingly, electric characteristics of the transistor can be improved and variation in the electric characteristics thereof can be reduced. The timing of this heat treatment is not particularly limited as long as it is after the formation of the insulating film 612, and this heat treatment can be performed without increasing the number of manufacturing steps by doubling as another step such as heat treatment for formation of a resin film or heat treatment for reduction of the resistance of a transparent conductive film, so that the oxide semiconductor film 605 can be made to be an i-type semiconductor film or a substantially i-type semiconductor film.

Next, after a conductive film is formed over the insulating film 612, a back gate electrode may be formed so as to overlap with the oxide semiconductor film 605 by patterning the conductive film. When the back gate electrode is formed, an insulating film is formed so as to cover the back gate electrode. The back gate electrode can be formed using a material and a structure similar to those of the gate electrode 601, the electrode 602, the source electrode 607, the drain electrode 608, the wiring 609, the wiring 610, or the wiring 611.

The thickness of the back gate electrode is 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, the back gate electrode may be formed in a such a manner that a conductive film in which a titanium film, an aluminum film, and a titanium film are stacked is formed, a resist mask is formed by a photolithography method or the like, and unnecessary portions are removed by etching so that the conductive film is processed (patterned) into a desired shape.

The insulating film is preferably formed using a material having a high barrier property which can prevent moisture, hydrogen, oxygen, and the like in an atmosphere from influencing characteristics of the transistor. For example, it is possible to form a single-layer structure or a stacked-layer structure of a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like, as an insulating film having a high barrier property, by a plasma CVD method, a sputtering method, or the like. In order to obtain an effect of a barrier property, the insulating film is preferably formed to a thickness of 15 nm to 400 nm, for example.

In this embodiment, an insulating film is formed to a thickness of 300 nm by a plasma CVD method. The insulating film is formed under the following conditions: the silane gas flow rate is 4 sccm; the flow rate of dinitrogen monoxide (N₂O) is 800 sccm; and the substrate temperature is 400° C.

Through the above-described steps, a transistor 620 that functions as a switching element, the n-channel transistor 520 and the p-channel transistor 521 that form a phase-inversion element, and a capacitor 623 are formed. Note that the capacitor 623 is formed in a region where the electrode 602 and the source electrode 607 overlap with each other with the gate insulating film 603 interposed therebetween. Note also that the capacitor 623 is not necessarily formed over the same layer as the transistor 620; for example, the capacitor 623 may be formed over the same layer as the n-channel transistor 520 and the p-channel transistor 521.

The transistor 620 includes the gate electrode 601, the gate insulating film 603 over the gate electrode 601, the oxide semiconductor film 605 which is over the gate insulating film 603 and overlaps with the gate electrode 601, and a pair of the source electrode 607 and the drain electrode 608 formed over the oxide semiconductor film 605. The transistor 620 may further include the insulating film 612 provided over the oxide semiconductor film 605 as its component. Note that the transistor 620 illustrated in FIG. 11B has a channel-etched structure in which part of the oxide semiconductor film 605 is exposed between the source electrode 607 and the drain electrode 608.

Although description is given using a single-gate transistor as the transistor 620, a multi-gate transistor including a plurality of channel formation regions by including a plurality of the gate electrodes 601 that are electrically connected to each other may be formed as needed.

Note that the energy band gap of the oxide semiconductor is 3 eV to 3.5 eV. The band gap of silicon carbide and the band gap of gallium nitride are 3.26 eV and 3.39 eV, respectively, which are about three times as large as that of silicon. Therefore, these compound semiconductors such as silicon carbide and gallium nitride are similar to the oxide semiconductor in that they are both wide band gap semiconductors. The characteristics of the wide band gap is advantageous for improving the withstand voltage, reducing the electric power loss of a semiconductor device, and the like.

However, the compound semiconductor such as silicon carbide or gallium nitride needs much higher process temperature or treatment temperature than the oxide semiconductor. The process temperature of silicon carbide is about 1500° C. and the process temperature of gallium nitride is about 1100° C., which does not allow film formation over a silicon wafer that can be obtained easily or a glass substrate whose allowable temperature limit is low. Therefore, an inexpensive substrate cannot be used, and further, the compound semiconductor cannot be applied when the size of a substrate is increased, so that the mass productivity of semiconductor devices using the compound semiconductor such as silicon carbide or gallium nitride is low. In contrast, the oxide semiconductor can be deposited in heat treatment at 300° C. to 850° C. so as to be deposited over a glass substrate. In addition, as described in this embodiment, it is possible to form a semiconductor element using an oxide semiconductor over an integrated circuit using a normal semiconductor material.

Next, how characteristics of the transistor are influenced by high purification of the oxide semiconductor film by removal of impurities such as moisture or hydrogen contained in the oxide semiconductor film as much as possible as in this embodiment will be described.

FIG. 19 is a cross-sectional view of a transistor including an oxide semiconductor. An oxide semiconductor film (OS) is provided over a gate electrode (GE) with a gate insulating film (GI) therebetween, and a source electrode (S) and a drain electrode (D) are provided thereover. An insulating film is formed over the source electrode (S) and the drain electrode (D).

FIG. 20 is an energy band diagram (a schematic diagram) along an A-A′ section illustrated in FIG. 19. In FIG. 20, a black circle () and a white circle (∘) represent an electron and a hole and have electric charges (−q, +q), respectively. The case where a positive voltage (V_(D)>0) is applied to the drain electrode (D) and no voltage (V_(G)=0) is applied to the gate electrode (GE) is shown by dashed lines, and the case where a positive voltage (V_(D)>0) is applied to the drain electrode (D) and a positive voltage (V_(G)>0) is applied to the gate electrode (GE) is shown by solid lines. In the case where voltage is not applied to the gate electrode (GE), a carrier (an electron) is not injected to the oxide semiconductor film (OS) side from the source electrode (S) owing to the high potential barrier, so that current does not flow, which means an OFF state. In contrast, when a positive voltage is applied to the gate electrode (GE), the potential barrier is decreased, so that current flows in the oxide semiconductor film (OS), which means an ON state.

FIGS. 21A and 21B are energy band diagrams (schematic diagrams) along a B-B′ section illustrated in FIG. 19. FIG. 21A illustrates a state where a positive voltage (V_(G)>0) is applied to a gate electrode (GE) and an ON state where carriers (electrons) flow between the source electrode and the drain electrode. FIG. 21B illustrates a state where a negative voltage (V_(G)<0) is applied to the gate electrode (GE) and the transistor is in an OFF state.

FIG. 22 illustrates relations between the vacuum level and the work function (φ_(M)) of a metal and between the vacuum level and the electron affinity (x) of an oxide semiconductor.

At normal temperature, electrons in the metal are degenerated and the Fermi level is located in the conduction band. On the other hand, in general, a conventional oxide semiconductor is an n-type semiconductor, and the Fermi level (Ef) thereof is located nearer the conduction band (Ec) away from an intrinsic Fermi level (Ei) which is located in the center of the band gap. Note that it is known that part of hydrogen in the oxide semiconductor serves as a donor and one of factors that make the oxide semiconductor an n-type semiconductor. In addition, oxygen deficiency is also known as one of the factors that make the oxide semiconductor have n-type conductivity.

In contrast, in one embodiment of the present invention, an oxide semiconductor is made to be an intrinsic (i-type) semiconductor or extremely close to an intrinsic semiconductor by removing hydrogen that is an n-type impurity from an oxide semiconductor, highly purifying the oxide semiconductor such that an impurity other than a main component of the oxide semiconductor is prevented from being contained therein as much as possible, and removing oxygen deficiency. That is, the oxide semiconductor becomes an i-type semiconductor not by addition of impurities but by removal of impurities such as moisture or hydrogen and oxygen deficiency as much as possible to have high purity, so that an oxide semiconductor which is an intrinsic (i-type) semiconductor or is extremely close to an intrinsic (i-type) semiconductor is obtained. With the above structure, the Fermi level (Ef) can be extremely close to the same level as the intrinsic Fermi level (Ei), as indicated by arrows.

It is said that the band gap (Eg) of an oxide semiconductor is 3.15 eV and that the electron affinity (χ) thereof is 4.3 V. In the case where titanium (Ti) is used as a material for forming the source electrode and the drain electrode, the work function of titanium (Ti) is substantially equal to the electron affinity (χ) of the oxide semiconductor. In that case, a Schottky barrier to electrons is not formed at an interface between the metal and the oxide semiconductor. There are materials that satisfy this condition besides titanium.

In this case, as shown in FIG. 21A, the electron moves along the lowest part of the oxide semiconductor, which is energetically stable, at an interface between the gate insulating film and the highly-purified oxide semiconductor.

In FIG. 21B, when a negative voltage is applied to the gate electrode (GE), holes which are minority carriers are substantially zero; therefore, current is extremely close to zero.

For example, even when an element has a channel width (W) of 1×10⁶ μm and a channel length (L) of 10 μm, OFF-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between the source electrode and the drain electrode of from 1 V to 10 V. According to measurement, a transistor can have a much lower OFF-state current density, which is from 10 zA/μm to 100 zA/μm, at a voltage between a source electrode and a drain electrode of the transistor of 3 V. In the measurement, a transistor including a highly-purified oxide semiconductor film and a 100-nm-thick gate insulating film was used for a switching element which stores charge of a storage capacitor, and the OFF-state current of the transistor was measured by a change in the amount of charge in the storage capacitor per a unit hour. That is, the OFF-state current density of the transistor in which the highly-purified oxide semiconductor film is used as an active layer can be less than or equal to 100 zA/μm, preferably less than or equal to 10 zA/μm, more preferably less than or equal to 1 zA/μm.

In this manner, the oxide semiconductor film is highly purified so that impurities such as moisture or hydrogen other than a main component of the oxide semiconductor are contained as little as possible, whereby the operation of the transistor can be favorable.

This embodiment can be implemented by being combined as appropriate with any of the above-described embodiments.

Embodiment 5

In this embodiment, a transistor including an oxide semiconductor film has a structure which is different from that of the transistor of Embodiment 4 is described.

In a manner similar to that in Embodiment 4, a memory device illustrated in FIG. 12A includes the n-channel transistor 520 and the p-channel transistor 521 each of which includes crystalline silicon. In addition, a bottom-gate transistor 630 which has a channel-protective structure and includes an oxide semiconductor film is formed over the n-channel transistor 520 and the p-channel transistor 521 in FIG. 12A.

The transistor 630 includes a gate electrode 631 provided over the insulating film 532, a gate insulating film 632 provided over the gate electrode 631, an oxide semiconductor film 633 which overlaps with the gate electrode 631 over the gate insulating film 632, a channel protective film 634 which is provided over the island-shaped oxide semiconductor film 633 to overlap with the gate electrode 631, and a source electrode 635 and a drain electrode 636 which are provided over the oxide semiconductor film 633. The transistor 630 may further include an insulating film 637 provided over the oxide semiconductor film 633 as its component.

The channel protective film 634 can prevent a portion of the oxide semiconductor film 633, which serves as a channel formation region, from being damaged in a later step (for example, reduction in thickness due to plasma or an etchant in etching). Therefore, reliability of the transistor can be improved.

An inorganic material containing oxygen (silicon oxide, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, or the like) can be used for the channel protective film 634. The channel protective film 634 can be formed by a vapor deposition method such as a plasma CVD method or a thermal CVD method, or a sputtering method. After the formation of the channel protective film 634, the shape thereof is processed by etching. Here, the channel protective film 634 is formed in such a manner that a silicon oxide film is formed by a sputtering method and processed by etching using a mask formed by photolithography.

By using the inorganic material containing oxygen for the channel protective film 634, even when oxygen deficiency is generated in the oxide semiconductor film 633 owing to heat treatment for reducing moisture or hydrogen, a region of the oxide semiconductor film 633 which is in contact with at least the channel protective film 634 can be supplied with oxygen and oxygen deficiency which serves as a donor can be reduced, so that a structure which satisfies the stoichiometric composition ratio can be obtained. Therefore, the channel formation region can be made to be an i-type semiconductor or a substantially i-type semiconductor, variation in electric characteristics of the transistor due to oxygen deficiency can be reduced, and the electric characteristics can be improved.

Note that the transistor 630 may further include a back gate electrode over the insulating film 637. The back gate electrode is formed so as to overlap with a channel formation region in the oxide semiconductor film 633. Further, the back gate electrode may be electrically insulated and in a floating state, or may be in a state where the back gate electrode is supplied with a potential. In the latter case, the back gate electrode may be supplied with the potential having the same level as the gate electrode 631, or may be supplied with a fixed potential such as a ground potential. The level of the potential supplied to the back gate electrode is controlled, whereby the threshold voltage of the transistor 630 can be controlled.

In a manner similar to that in Embodiment 4, a memory device illustrated in FIG. 12B includes the n-channel transistor 520 and the p-channel transistor 521 each of which includes crystalline silicon. In addition, a bottom-contact transistor 640 including an oxide semiconductor film is formed over the n-channel transistor 520 and the p-channel transistor 521 in FIG. 12B.

The transistor 640 includes a gate electrode 641 provided over the insulating film 532, a gate insulating film 642 provided over the gate electrode 641, a source electrode 643 and a drain electrode 644 which are provided over the gate insulating film 642, and an oxide semiconductor film 645 which overlaps with the gate electrode 641. The transistor 640 may further include an insulating film 646 provided over the oxide semiconductor film 645 as its component.

In the case of the bottom-contact transistor 640 as illustrated in FIG. 12B, the thickness of each of the source electrode 643 and the drain electrode 644 is preferably smaller than that of the bottom-gate transistor described in Embodiment 4 in order to prevent disconnection of the oxide semiconductor film 645 formed later. Specifically, the thicknesses of the source electrode 643 and the drain electrode 644 are 10 nm to 200 nm, preferably 50 nm to 75 nm.

Note that the transistor 640 may further include a back gate electrode over the insulating film 646. The back gate electrode is formed so as to overlap with a channel formation region in the oxide semiconductor film 645. Further, the back gate electrode may be electrically insulated and in a floating state, or may be in a state where the back gate electrode is supplied with a potential. In the latter case, the back gate electrode may be supplied with the potential having the same level as the gate electrode 641, or may be supplied with a fixed potential such as a ground potential. The level of the potential supplied to the back gate electrode is controlled, whereby the threshold voltage of the transistor 640 can be controlled.

In a manner similar to that in Embodiment 4, a memory device illustrated in FIG. 12C includes the n-channel transistor 520 and the p-channel transistor 521 each of which includes crystalline silicon. In addition, a top-gate transistor 650 including an oxide semiconductor film is provided over the n-channel transistor 520 and the p-channel transistor 521 in FIG. 12C.

The transistor 650 includes a source electrode 651 and a drain electrode 652 which are provided over the insulating film 532, an oxide semiconductor film 653 which is provided over the source electrode 651 and the drain electrode 652, a gate insulating film 654 provided over the oxide semiconductor film 653, and a gate electrode 655 which overlaps with the oxide semiconductor film 653 over the gate insulating film 654. In addition, the transistor 650 may further include an insulating film 656 provided over the gate electrode 655 as its component.

In the case of the top-gate transistor 650 as illustrated in FIG. 12C, the thickness of each of the source electrode 651 and the drain electrode 652 is preferably smaller than that of the bottom-gate transistor described in Embodiment 4 in order to prevent disconnection of the oxide semiconductor film 653 formed later. Specifically, the thicknesses of the source electrode 651 and the drain electrode 652 are 10 nm to 200 nm, preferably 50 nm to 75 nm.

In the memory device which is one embodiment of the present invention, a phase-inversion element, a switching element, and the like may include a transistor which is manufactured using a bulk single crystal semiconductor substrate. In FIG. 23, a cross-sectional view of a memory device in which a transistor including an oxide semiconductor is formed over a transistor which is formed using the bulk single crystal semiconductor substrate is illustrated as an example.

The memory device illustrated in FIG. 23 includes an n-channel transistor 661 and a p-channel transistor 662 which are provided over a semiconductor substrate 660, a transistor 664 which is provided over an insulating film 663 covering the n-channel transistor 661 and the p-channel transistor 662 and which is used as a capacitor switching element, and a capacitor 665.

Although the case where the transistor 664 includes an oxide semiconductor in a channel formation region and has a structure described in Embodiment 4 is described as an example, the structure may be that illustrated in FIGS. 12A to 12C.

The semiconductor substrate 660 can be, for example, a single crystal silicon substrate having n-type or p-type conductivity, a compound semiconductor substrate (e.g., a GaAs substrate, an InP substrate, a GaN substrate, a SiC substrate, a sapphire substrate, or a ZnSe substrate), or the like. In FIG. 23, the case where a single crystal silicon substrate having n-type conductivity is used is illustrated as an example.

In addition, the n-channel transistor 661 and the p-channel transistor 662 are electrically isolated by an element isolation insulating film 666. For formation of the element isolation insulating film 666, a selective oxidation method (local oxidation of silicon (LOCOS) method), a trench isolation method, or the like can be used.

In a region where the p-channel transistor 662 is formed, a region called a p-well 667 is formed by selectively introducing an impurity element imparting p-type conductivity. In the case where a semiconductor substrate having p-type conductivity is used, an impurity element imparting n-type conductivity may be selectively introduced to a region where the n-channel transistor 661 is formed, so that an n-well may be formed.

This embodiment can be implemented by being combined as appropriate with any of the above-described embodiments.

Embodiment 6

In this embodiment, a structure of a memory device which is one embodiment of the present invention will be described. In the memory device, a transistor including a highly-purified oxide semiconductor in a channel formation region is used for a switching element which controls supply of a power supply potential to a memory element.

In FIG. 13A, the structure of a memory device of this embodiment is illustrated as an example. The memory device illustrated in FIG. 13A includes a switching element 401 and a memory element group 403 including a plurality of memory elements 402. Specifically, as each of the memory elements 402, a memory element having any of the structures described in Embodiments 1 to 5 can be used. Each of the memory elements 402 included in the memory element group 403 is supplied with a high-level power supply potential VDD via the switching element 401. Further, each of the memory elements 402 included in the memory element group 403 is supplied with a potential of a signal IN and a low-level power supply potential VSS.

In FIG. 13A, a transistor including an oxide semiconductor in a channel formation region is used for the switching element 401, and the switching of the transistor is controlled by a signal Sig A supplied to a gate electrode thereof. Since the transistor used for the switching element 401 includes a highly-purified oxide semiconductor in a channel formation region, OFF-state current is extremely low as described above.

Note that in FIG. 13A, a structure in which the switching element 401 includes only one transistor is illustrated; however, the present invention is not limited to this structure. In one embodiment of the present invention, the switching element 401 may include a plurality of transistors. In the case where the plurality of transistors which serve as switching elements are included in the switching element 401, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

Although the switching element 401 controls the supply of the high-level power supply potential VDD to each of the memory elements 402 included in the memory element group 403 in FIG. 13A, the switching element 401 may control the supply of the low-level power supply potential VSS. In FIG. 13B, an example of a memory device in which each of the memory elements 402 included in the memory element group 403 is supplied with the low-level power supply potential VSS via the switching element 401 is illustrated. The supply of the low-level power supply potential VSS to each of the memory elements 402 included in the memory element group 403 can be controlled by the switching element 401.

Next, a structure of a transistor for a power device capable of controlling higher voltage or higher current than the transistor including an oxide semiconductor in a channel formation region illustrated in Embodiment 4 or Embodiment 5 will be described. By using a transistor having such a structure for the switching element 401, the reliability of the memory device can be further increased. Note that the same portions as Embodiment 4 or Embodiment 5 or portions having functions similar to those of Embodiment 4 or Embodiment 5 can be formed as in Embodiment 4 or Embodiment 5, and also the same steps as Embodiment 4 or Embodiment 5 or the steps similar to those of Embodiment 4 or Embodiment 5 can be performed in a manner similar to those of Embodiment 4 or Embodiment 5; therefore, repetitive description thereof is omitted.

In FIG. 14A, a cross-sectional view of a transistor 420 described in this embodiment is illustrated. FIG. 14B is a top view of the transistor 420. A cross-sectional view along a dashed line B1-B2 in FIG. 14B corresponds to FIG. 14A.

The transistor 420 includes a first electrode 421 over an insulating surface.

The first electrode 421 is formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten, and yttrium; an alloy containing any of these metal elements as a component; an alloy containing these metal elements in combination; or the like. Alternatively, one or more metal elements selected from manganese, magnesium, zirconium, and beryllium can be used. In addition, the first electrode 421 can have a single-layer structure or a stacked structure having two or more layers. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a tungsten film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in the order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used.

The first electrode 421 can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. In addition, the first electrode 421 can have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element.

The first electrode 421 can be formed in such a manner that a conductive film is formed over the insulating surface by a sputtering method, a CVD method, or a vacuum evaporation method, a resist mask is formed over the conductive film in a photolithography process, and the conductive film is etched using the resist mask. Alternatively, the first electrode 421 is formed by a printing method or an inkjet method without using a photolithography process, so that the number of steps can be reduced. Note that end portions of the first electrode 421 preferably have a tapered shape, so that the coverage with a gate insulating film formed later is improved. When the angle between the end portion of the first electrode 421 and the insulating surface over which the first electrode 421 is formed is greater than or equal to 30° and less than or equal to 60°, preferably greater than or equal to 40° and less than or equal to 50°, the coverage with the gate insulating film formed later can be improved.

In this embodiment, as the conductive film for forming the first electrode 421, a 50-nm-thick titanium film is formed by a sputtering method, a 100-nm-thick aluminum film is formed, and a 50-nm-thick titanium film is formed. Next, etching is performed using the resist mask formed in the photolithography process, whereby the first electrode 421 is formed. By forming a resist mask using an inkjet method instead of the resist mask formed in the photolithography process, the number of steps can be reduced.

The transistor 420 includes an island-shaped oxide semiconductor film 422 over the first electrode 421. The oxide semiconductor film 422 can be formed by a sputtering method, a coating method, a printing method, or the like. In this embodiment, after an oxide semiconductor film is formed by a sputtering method over the first electrode 421, the oxide semiconductor film is processed into a desired shape by etching or the like, so that the island-shaped oxide semiconductor film 422 is formed. Moreover, the oxide semiconductor film can be formed by a sputtering method under a rare gas (for example, argon) atmosphere, an oxygen atmosphere, or an atmosphere including a rare gas (for example, argon) and oxygen.

Note that the etching for forming the island-shaped oxide semiconductor film 422 may be performed in accordance with description of the etching of the oxide semiconductor film in Embodiment 4. Note that the angle formed between the end portion of the island-shaped oxide semiconductor film 422 which is formed by etching and the first electrode 421 is preferably greater than or equal to 30° and less than or equal to 60°, more preferably greater than or equal to 40° and less than or equal to 50°, whereby the coverage with the gate insulating film that is formed later can be improved.

Note that before the oxide semiconductor film is formed by sputtering, dust on a surface of the first electrode 421 is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering means a method in which a surface of a substrate is modified by application of voltage to the substrate side, not to a target side, in an argon atmosphere with the use of an RF power source and by generating plasma in the vicinity of the substrate. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used.

For the oxide semiconductor film 422, the above-described oxide semiconductor can be used.

In this embodiment, an In—Ga—Zn—O-based non-single-crystal film with a thickness of 30 nm, which is obtained by a sputtering method using an oxide semiconductor target containing indium (In), gallium (Ga), and zinc (Zn), is used as the oxide semiconductor film 422. As the target, a metal oxide target containing each metal atom at a composition ratio of In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used, for example. In addition, the oxide semiconductor film can be formed by a sputtering method in a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas (typically, argon) and oxygen. In the case of using a sputtering method, a target containing SiO₂ at greater than or equal to 2 wt % and less than or equal to 10 wt % may be used for forming a film. The fill rate of the metal oxide target containing In, Ga, and Zn is higher than or equal to 90% and lower than or equal to 100%, preferably higher than or equal to 95% and lower than or equal to 99.9%. With the use of a metal oxide target with high fill rate, the oxide semiconductor film formed has high density.

The substrate is held in a treatment chamber kept under reduced pressure, a sputtering gas from which hydrogen and moisture are removed is introduced into the treatment chamber from which remaining moisture is being removed, and the oxide semiconductor film 422 is formed over a substrate with the use of a metal oxide as a target. In film formation, the substrate temperature may be higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Film formation is performed while the substrate is heated, whereby the impurity concentration in the oxide semiconductor film formed can be reduced. In addition, damage by sputtering can be reduced. In order to remove moisture remaining in the treatment chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo pump provided with a cold trap. In the treatment chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), (more preferably, also a compound containing a carbon atom), and the like are removed, whereby the impurity concentration in the oxide semiconductor film formed in the treatment chamber can be reduced.

In this embodiment, as an example of a film formation condition of the oxide semiconductor film, the following conditions are applied: the substrate temperature is room temperature, the distance between the substrate and the target is 110 mm; the pressure is 0.4 Pa; the direct current (DC) power source is 0.5 kW; and the atmosphere contains oxygen and argon (oxygen flow rate of 15 sccm, argon flow rate of 30 sccm). Note that a pulse direct current (DC) power source is preferable because dust called particles generated in film formation can be reduced and the film thickness can be uniform. The thickness of the oxide semiconductor film is greater than or equal to 1 μm, preferably greater than or equal to 3 μm, more preferably greater than or equal to 10 μm. Note that the preferable thickness varies depending on an oxide semiconductor film material; thus, the appropriate thickness may be determined depending on a material.

Note that in order that hydrogen, hydroxyl group, and moisture are contained as little as possible in the oxide semiconductor film 422, it is preferable that the substrate over which layers up to and including the first electrode 421 are formed be preheated in a preheating chamber of the sputtering apparatus, so that impurities such as hydrogen or moisture absorbed onto the substrate is eliminated and evacuated, as pretreatment before film formation. The temperature for the preheating is higher than or equal to 100° C. and less than or equal to 400° C., preferably higher than or equal to 150° C. and less than or equal to 300° C. As an evacuation unit provided in the preheating chamber, a cryopump is preferable. Note that this preheating treatment can be omitted. Note also that this preheating may be similarly performed on the substrate over which layers up to and including the gate electrode are formed before the formation of the insulating film.

Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power supply, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case where an insulating film is formed, and a DC sputtering method is mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber.

Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge can be used.

Further, as a deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during film formation to form a thin compound film thereof, or a bias sputtering method in which voltage is also applied to a substrate during film formation can be used.

Note that heat treatment is performed on the oxide semiconductor film 422 in a reduced-pressure atmosphere, an atmosphere of an inert gas such as nitrogen and a rare gas, an oxygen atmosphere, or an ultra-dry air atmosphere (the moisture amount is 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, more preferably 10 ppb or less, in the case where measurement is performed by a dew point meter in a cavity ring down laser spectroscopy (CRDS) method). When the heat treatment is performed on the oxide semiconductor film 422, an oxide semiconductor film 422 in which moisture or hydrogen is eliminated is formed. Specifically, heat treatment may be performed at higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to the strain point of a glass substrate). Since dehydration or dehydrogenation can be performed in a short time with the RTA method, the heat treatment can be performed even at a temperature over the strain point of a glass substrate. In this embodiment, heat treatment is performed on the oxide semiconductor film 422 at a substrate temperature of 450° C. for 1 hour in a nitrogen atmosphere with the use of an electrical furnace that is one of heat treatment apparatuses, and then, the oxide semiconductor film is not exposed to air, and water and hydrogen are prevented from entering. Impurities such as moisture or hydrogen is eliminated by the heat treatment, so that the oxide semiconductor film 422 becomes an i-type (intrinsic) semiconductor or a substantially i-type semiconductor; therefore, deterioration of characteristics of the transistor due to the impurities, such as shifts in threshold voltage, can be prevented from being promoted and OFF-state current can be reduced.

Detailed description of the heat treatment apparatus used for the heat treatment is already made in Embodiment 4 and therefore omitted here.

Note that it is preferable that in the heat treatment, moisture, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. It is preferable that the purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower).

When impurities are added to an oxide semiconductor, in a gate bias-temperature stress test (BT test, the test condition is for example at 85° C. with 2×10⁶ V/cm for 12 hours), a bond between the impurities and a main component of the oxide semiconductor is broken by a high electrical field (B: bias) and high temperature (T: temperature), and a dangling bond generated causes drift of the threshold voltage (Vth). However, in the above-described manner, by improving the interface characteristics between the gate insulating film and the oxide semiconductor film and removing impurities, particularly hydrogen, water, and the like, in the oxide semiconductor film as much as possible, a transistor which remains stable even with respect to the BT test can be obtained.

Through the above-described steps, the hydrogen concentration in the oxide semiconductor film can be reduced and the oxide semiconductor film can be highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature of lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that productivity can be increased. In addition, by using the oxide semiconductor film in which the hydrogen concentration is reduced and purity is improved, it is possible to manufacture a transistor with high withstand voltage, a reduced short-channel effect, and a high on-off ratio.

The transistor 420 also includes a second electrode 423 over the oxide semiconductor film 422. The material and structure of the conductive film used for the second electrode 423 can be similar to those for the first electrode 421. In addition, a method for forming the second electrode 423 can be similar to that for the first electrode 421.

In this embodiment, a resist mask is formed over the conductive film which serves as the second electrode 423 in a photolithography process, and the conductive film is etched using the resist mask, so that the second electrode 423 is formed. Here, as the conductive film for forming the second electrode 423, a 50-nm-thick titanium film, a 100-nm-thick aluminum film, and a 50-nm-thick titanium film are stacked in that order. When the angle formed between end portion of the second electrode 423 and the oxide semiconductor film 422 is preferably greater than or equal to 30° and less than or equal to 60°, more preferably greater than or equal to 40° and less than or equal to 50°, the coverage with a gate insulating film formed later can be improved. In addition, the second electrode 423 is formed away from the first electrode 421 so as not to be in contact with the first electrode 421.

One of the first electrode 421 and the second electrode 423 functions as a source electrode of a transistor, and the other functions as a drain electrode thereof.

Heat treatment may be performed after the formation of the second electrode 423. The temperature of the heat treatment is higher than or equal to 400° C. and lower than or equal to 850° C., preferably higher than or equal to 400° C. and lower than the strain point of the substrate. In this embodiment, the substrate is introduced into an electric furnace which is one of heat treatment apparatuses, and heat treatment is performed on the oxide semiconductor film 422 in an inert gas atmosphere, such as a nitrogen atmosphere or a rare gas atmosphere, at 450° C. for 1 hour, and then the oxide semiconductor film is not exposed to air. Accordingly, hydrogen, water, hydroxyl group, hydride, or the like can be prevented from being mixed into the oxide semiconductor film, hydrogen concentration is further reduced, and the oxide semiconductor film is highly purified, whereby an i-type oxide semiconductor film or a substantially i-type oxide semiconductor film can be obtained.

Note that it is preferable that in the heat treatment, hydrogen, water, hydroxyl group, hydride, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. Alternatively, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is preferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is, the concentration of the impurities is 1 ppm or lower, preferably 0.1 ppm or lower).

The transistor 420 also includes a gate insulating film 424 covering the first electrode 421, the oxide semiconductor film 422, and the second electrode 423, and a gate electrode 425 which is formed over the gate insulating film 424. The gate insulating film 424 is formed using a film having a single layer or a stacked layer which includes one or more of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, a hafnium oxide film, and a tantalum oxide film by plasma CVD, sputtering, or the like.

When the gate insulating film 424 is formed using a high-k material such as hafnium silicate (HfSiOx), HfSixOy to which N is added, hafnium aluminate (HfAlOx) to which N is added, hafnium oxide, or yttrium oxide, gate leakage current can be reduced. Further, a stacked structure in which a high-k material and one or more of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, and an aluminum oxide film are stacked can be used. The thickness of the gate insulating film 424 is preferably greater than or equal to 50 nm and less than or equal to 500 nm. When the thickness of the gate insulating film 424 is large, the gate leakage current can be reduced.

It is preferable that the gate insulating film 424 contain impurities such as moisture or hydrogen as little as possible. In the case where a silicon oxide film is formed by a sputtering method, a silicon target or a quartz target is used as a target and a mixed gas of oxygen and argon is used as a sputtering gas.

The oxide semiconductor that is made to be an intrinsic oxide semiconductor or a substantially intrinsic oxide semiconductor (the oxide semiconductor that is highly purified) by removal of impurities is extremely sensitive to an interface state and an interface electric charge; thus, an interface between the oxide semiconductor and the gate insulating film 424 is important. Therefore, the gate insulating film (GI) that is in contact with the highly-purified oxide semiconductor needs to have a higher quality.

For example, high-density plasma CVD using microwaves (2.45 GHz) is preferable because a dense high-quality insulating film having high withstand voltage can be formed. This is because when the highly-purified oxide semiconductor is closely in contact with the high-quality gate insulating film, the interface state can be reduced and interface properties can be favorable.

Needless to say, another method such as sputtering method or plasma CVD method can be employed as long as the method enables formation of a good-quality insulating film as a gate insulating film 424. In addition, any insulating film can be used as long as film quality and interface properties with an oxide semiconductor of the gate insulating film 424 are modified by heat treatment performed after deposition. In any case, any insulating film can be used as long as film quality as a gate insulating film is high, interface state density with an oxide semiconductor is decreased, and a favorable interface can be formed.

The gate insulating film 424 may have a structure in which an insulating film formed using a material having a high barrier property and an insulating film having a lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film are stacked. In this case, the insulating film such as a silicon oxide film or a silicon oxynitride film is formed between the insulating film having a barrier property and the oxide semiconductor film. As the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be given, for example. The insulating film having a barrier property is used, so that impurities in an atmosphere, such as moisture or hydrogen, or impurities contained in the substrate, such as an alkali metal or a heavy metal, can be prevented from entering the oxide semiconductor film, the gate insulating film 424, or at the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. In addition, the insulating film having a lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film is formed so as to be in contact with the oxide semiconductor film, so that the insulating film formed using a material having a high barrier property can be prevented from being in direct contact with the oxide semiconductor film.

For example, a gate insulating film having a total thickness of 100 nm may be formed in such a manner that a silicon oxide film (SiO_(x) (x>0)) with a thickness of greater than or equal to 5 nm and less than or equal to 300 nm is formed as a first gate insulating film and a silicon nitride film (SiN_(y) (y>0)) with a thickness of greater than or equal to 50 nm and less than or equal to 200 nm is stacked over the first gate insulating film as a second gate insulating film by sputtering. In this embodiment, a 100-nm-thick silicon oxide film is formed by an RF sputtering method under a condition that the pressure is 0.4 Pa, the high-frequency power is 1.5 kW, and an atmosphere containing oxygen and argon (an oxygen flow rate of 25 sccm: an argon flow rate of 25 sccm=1:1) is used.

Note that in order that hydrogen, hydroxyl group, and moisture are contained as little as possible in the gate insulating film 424, it is preferable that the substrate over which first electrode 421, the oxide semiconductor film 422 and the second electrode 423 are formed be preheated in a preheating chamber of the sputtering apparatus, so that impurities such as hydrogen or moisture absorbed onto the substrate is eliminated and evacuated, as pretreatment before film formation. The temperature for the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an evacuation unit provided in the preheating chamber, a cryopump is preferable. Note that this preheating treatment can be omitted.

Note that after the gate insulating film 424 is formed, heat treatment may be performed. The heat treatment is performed in an air atmosphere or an inert gas atmosphere (nitrogen, helium, neon, argon, or the like) at higher than or equal to 200° C. and lower than or equal to 400° C., for example, at higher than or equal to 250° C. and lower than or equal to 350° C. In this embodiment, for example, heat treatment for 1 hour at 250° C. in a nitrogen atmosphere is performed. By the heat treatment performed in the state where silicon oxide contained in the gate insulating film 424 is in contact with the oxide semiconductor film 422, oxygen is supplied from the silicon oxide even when an oxygen deficiency is generated in the above-described heat treatment for eliminating moisture or hydrogen, whereby the oxygen deficiency which forms donors can be reduced, a structure which satisfies the stoichiometric composition ratio can be obtained, and the oxide semiconductor film 422 is made to be an i-type semiconductor film or a substantially i-type semiconductor film. There is no particular limitation on the timing of this heat treatment as long as it is after the formation of the gate insulating film 424. The heat treatment may be performed after another step, for example, after any one of the gate electrode 425, an insulating film 426, and wirings 434, 435, and 436 is formed. This heat treatment can be performed without increasing the number of manufacturing steps by doubling as another step such as heat treatment for reduction of the resistance of a transparent conductive film.

The gate electrode 425 can be formed using a single layer or a stacked layer of one or more conductive films including a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, neodymium, or scandium, or an alloy material which includes any of these metal materials as a main component; or a nitride of any of these metals. Note that aluminum or copper can also be used as such metal materials as long as aluminum or copper can withstand a temperature of heat treatment performed in a later process. Aluminum or copper is preferably combined with a refractory metal material so as to prevent a heat resistance problem and a corrosive problem. As the refractory metal material, molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium, or the like can be used.

For example, as a two-layer structure of the gate electrode 425, the following structures are preferable: a two-layer structure in which a molybdenum film is stacked over an aluminum film, a two-layer structure in which a molybdenum film is stacked over a copper film, a two-layer structure in which a titanium nitride film or a tantalum nitride film is stacked over a copper film, and a two-layer structure in which a titanium nitride film and a molybdenum film are stacked. As a three-layer structure of the gate electrode 425, the following structure is preferable: a stacked structure containing an aluminum film, an alloy film of aluminum and silicon, an alloy film of aluminum and titanium, or an alloy film of aluminum and neodymium in a middle layer and any of a tungsten film, a tungsten nitride film, a titanium nitride film, and a titanium film in a top layer and a bottom layer.

Further, by using a light-transmitting oxide conductive film of indium oxide, indium tin oxide, an indium oxide-zinc oxide alloy, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, or the like for the gate electrode 425, the aperture ratio of a pixel portion can be increased.

The gate electrode 425 is formed to a thickness of 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, after a conductive film with a thickness of 150 nm for the gate electrode is formed by a sputtering method using a tungsten target, the conductive film is processed (patterned) by etching to have a desired shape, so that the gate electrode 425 is formed. The gate electrode 425 is formed at least so as to overlap with the end portion of the oxide semiconductor film 422 with the gate insulating film 424 provided therebetween. In the end portion of the oxide semiconductor film 422, a channel formation region is formed in a portion which overlaps with the gate electrode 425 with the gate insulating film 424 provided therebetween. Note that the gate electrode 425 preferably has a tapered shape because coverage with the insulating film 426 stacked thereover can be improved. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

The transistor 420 also includes the insulating film 426 covering the first electrode 421, the oxide semiconductor film 422, the second electrode 423, the gate insulating film 424, and the gate electrode 425. The insulating film 426 preferably contains impurities such as moisture or hydrogen as little as possible, and may be formed using a single-layer insulating film or a plurality of insulating films stacked. The insulating film 426 is formed using, for example, an oxide insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or an aluminum oxynitride film; or a nitride insulating film such as a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film. Alternatively, an oxide insulating film and a nitride insulating film can be stacked. An insulating film having a high barrier property, for example, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film may be used for the insulating film 426, so that impurities such as moisture or hydrogen can be prevented from entering the oxide semiconductor film 422, the gate insulating film 424, or the interface between the oxide semiconductor film 422 and another insulating film and the vicinity thereof.

In this embodiment, the insulating film 426 is formed to have a structure in which a 100-nm-thick silicon nitride film formed by a sputtering method is stacked over a 200-nm-thick silicon oxide film formed by a sputtering method. Note that when the insulating film 426 is formed by a sputtering method, the substrate is heated to a temperature of 100° C. to 400° C., a sputtering gas in which hydrogen, water, hydroxyl group, hydride, or the like is removed and which contains high-purity nitrogen is introduced, and the insulating film 426 may be formed using a silicon semiconductor target. Also in this case, an insulating film is preferably formed in the state where hydrogen, water, hydroxyl group, hydride, or the like remaining in the treatment chamber is removed.

Note that after the insulating film 426 is formed, heat treatment may be performed. The heat treatment is performed in an inert gas atmosphere (nitrogen, helium, neon, argon, or the like) at higher than or equal to 200° C. and lower than or equal to 400° C., for example, at higher than or equal to 250° C. and lower than or equal to 350° C.

Contact holes 431, 432, and 433 are formed as follows: a resist mask is formed by a photolithography process and parts of the gate insulating film 424 and the insulating film 426 are selectively etched using the resist mask. A part of the gate electrode 425 is exposed in the contact hole 431; a part of the second electrode 423 is exposed in the contact hole 432; and a part of the gate electrode 425 is exposed in the contact hole 433. At the time of the formation of these contact holes, a contact hole so as to expose the first electrode 421 may be formed in a region of the first electrode 421, which is not covered with the gate electrode 425.

The wiring 434, the wiring 435, and the wiring 436 are connected to the gate electrode 425, the second electrode 423, and the gate electrode 425 through the contact hole 431, the contact hole 432, and the contact hole 433 which are formed in the insulating film 426, respectively. Note that in formation of these wirings, a wiring connected to the first electrode 421 through a contact hole may be formed.

The wirings 434, 435, and 436 can be formed using a conductive film having a structure and a material which are similar to those for the first electrode 421 by a manufacturing method similar to that for the first electrode 421.

In FIG. 14B, a wiring 440 is a wiring formed at the same time as the wirings 434, 435, and 436 and is connected to the first electrode 421 through a contact hole 441.

Through the above-described steps, the hydrogen concentration in the oxide semiconductor film can be reduced and the oxide semiconductor film can be highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature of lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that productivity can be increased. In addition, by using the oxide semiconductor film in which the hydrogen concentration is reduced and purity is improved, it is possible to manufacture a transistor with high withstand voltage, a reduced short-channel effect, and a high on-off ratio.

Although all the part of the oxide semiconductor film 422, which is formed in a region which is different from the region where the second electrode 423 is formed is covered with the gate electrode 425 in this embodiment, the present invention is not limited to this structure as long as at least a part of the portion of the oxide semiconductor film 422, which is formed in a region different from the region where the second electrode 423 is formed, is covered with the gate electrode 425.

Here, the drain withstand voltage of the transistor described in this embodiment is described below.

When the electric field in the semiconductor reaches a certain threshold value, impact ionization occurs, carriers accelerated by the high electric field impact crystal lattices in a depletion layer, thereby generating pairs of electrons and holes. When the electric field becomes even higher, the pairs of electrons and holes generated by the impact ionization are further accelerated by the electric field, and the impact ionization is repeated, resulting in an avalanche breakdown in which current is increased exponentially. The impact ionization occurs because carriers (electrons and holes) have kinetic energy that is larger than or equal to the band gap of the semiconductor. Therefore, as the band gap is larger, the electric field needed to cause the impact ionization is higher.

Since the band gap of the oxide semiconductor is 3.15 eV, which is larger than the band gap of amorphous silicon, 1.74 eV, the avalanche breakdown is expected to be unlikely to occur. Therefore, a transistor including the oxide semiconductor has a high drain withstand voltage, and an exponential sudden increase of ON-state current is expected to be unlikely to occur when a high electric field is applied.

Next, hot-carrier degradation of a transistor including an oxide semiconductor will be described.

The hot-carrier degradation means deterioration of transistor characteristics, e.g., variation in the threshold voltage or leakage current, which is caused as follows: electrons that are accelerated to be rapid are injected into a gate insulating film in the vicinity of a drain in a channel and become fixed electric charge or form trap levels at the interface between the gate insulating film and the oxide semiconductor. The factors of the hot-carrier degradation are, for example, channel-hot-electron injection (CHE injection) and drain-avalanche-hot-carrier injection (DAHC injection).

Since the band gap of silicon is narrow, electrons are likely to be generated like an avalanche owing to an avalanche breakdown, and electrons that are accelerated to be so rapid as to go over a barrier to the gate insulating film are increased in number. However, the oxide semiconductor described in this embodiment has a wide band gap; therefore, the avalanche breakdown is unlikely to occur and resistance to the hot-carrier degradation is higher than that of silicon. Note that although the band gap of silicon carbide which is one of materials having high withstand voltage and that of an oxide semiconductor are substantially equal to each other, electrons are less likely to be accelerated, hot-carrier degradation is less likely to be caused than in the case of silicon carbide, and drain withstand voltage is high in the oxide semiconductor because the mobility of the oxide semiconductor is lower than that of silicon carbide by approximately two orders of magnitude.

From the above, a transistor including an oxide semiconductor has high drain withstand voltage; specifically, such a transistor can have a drain withstand voltage of greater than or equal to 100 V, preferably greater than or equal to 500 V, more preferably greater than or equal to 1 kV.

Comparison between a transistor using silicon carbide, which is a typical example of a transistor, and a transistor using an oxide semiconductor will be described below. Here, 4H—SiC is used as the silicon carbide.

An oxide semiconductor and 4H—SiC have some things in common. One example is intrinsic carrier density. Using the Fermi-Dirac distribution at normal temperature, the intrinsic carrier density of the oxide semiconductor is estimated to approximately 10⁻⁷ cm⁻³, which is extremely low like the carrier density of 4H—SiC, i.e., 6.7×10⁻¹¹ cm⁻³.

In addition, the energy band gap of the oxide semiconductor is 3.0 eV to 3.5 eV and that of 4H—SiC is 3.26 eV, which means that both the oxide semiconductor and the silicon carbide are wide-gap semiconductors.

However, the manufacturing temperature of a transistor including an oxide semiconductor and the manufacturing temperature of a transistor including silicon carbide are quite different. Silicon carbide generally needs heat treatment at 1500° C. to 2000° C. In contrast, an oxide semiconductor can be formed by heat treatment at 300° C. to 850° C., which allows a transistor to be manufactured over a large-sized substrate. In addition, throughput can be improved.

Further, a manufacturing process of the transistor including silicon carbide, which uses a PN junction, involves a step of doping with an impurity element that can be a donor or an acceptor (e.g., phosphorus or boron); therefore, the number of manufacturing steps is increased. On the other hand, the transistor including an oxide semiconductor is not needed to be provided with a PN junction; therefore, the number of manufacturing steps can be decreased and the throughput can be improved, and further, a large-sized substrate can be used.

Note that considerable research has been done on properties of oxide semiconductors such as density of states (DOS) in the band gap; however, the research does not include the idea of sufficiently reducing the DOS itself. In this embodiment, a highly-purified oxide semiconductor is formed by removing water or hydrogen which might induce the DOS from the oxide semiconductor. This is based on the idea of sufficiently reducing the DOS itself. Such a highly-purified oxide semiconductor enables fabrication of very excellent industrial products.

Further, it is also possible to form a more highly-purified (i-type) oxide semiconductor by supplying oxygen to a dangling bond of metal which is generated by oxygen vacancy and reducing the DOS due to the oxygen defects. For example, an oxide film containing excessive oxygen is formed in close contact with a channel formation region and then oxygen is supplied to the channel formation region from the oxide film, so that the DOS due to oxygen defects can be reduced.

It can be considered that a factor of a defect of an oxide semiconductor is a shallow level due to excessive hydrogen at 0.1 eV to 0.2 eV below the conduction band, a deep level resulted from lack of oxygen, or the like. Thorough removal of hydrogen and sufficient supply of oxygen for elimination of such a defect would be right as a technical thought.

An oxide semiconductor is generally considered as an n-type semiconductor; however, in this embodiment, an i-type oxide semiconductor is realized by removing impurities, particularly water or hydrogen. In this point, the technical thought of the present invention is novel as compared with a conventional technical thought in which an i-type semiconductor is formed by adding impurities to a semiconductor such as silicon.

By making the oxide semiconductor be an i-type oxide semiconductor, favorable temperature characteristics of the transistor can be obtained; specifically, in terms of the current vs. voltage characteristics of the transistor, ON-state current, OFF-state current, field-effect mobility, an S value, and a threshold voltage are hardly fluctuated at a temperature ranging from −25° C. to 150° C., and the current vs. voltage characteristics are hardly degraded by the temperature.

In the transistor using an oxide semiconductor which is described in this embodiment, mobility is lower than that in a transistor using silicon carbide by approximately two orders of magnitude; however, current value of the transistor and device characteristics can be improved by increasing the drain voltage and the channel width (W).

A technical idea of this embodiment is that a substance is not added to an oxide semiconductor and on the contrary the oxide semiconductor itself is highly-purified by intentionally removing impurities such as water or hydrogen which undesirably exist therein. In other words, the oxide semiconductor is highly-purified by removing water or hydrogen which forms a donor level, reducing oxygen deficiency, and sufficiently supplying oxygen that is a component of the oxide semiconductor.

At the time of deposition, the oxide semiconductor includes hydrogen at approximately 10²⁰ cm⁻³ which is measured by secondary ion mass spectrometry (SIMS). The oxide semiconductor is highly purified and made to be an electrically i-type (intrinsic) semiconductor by intentionally removing water or hydrogen which forms a donor level and further by adding oxygen (one of components of the oxide semiconductor), which is reduced at the same time as removal of water or hydrogen, to the oxide semiconductor.

In this embodiment, the amount of water and hydrogen in the oxide semiconductor is preferably as small as possible, and the number of carriers in the oxide semiconductor is preferably as small as possible. Specifically, a carrier density of lower than 1×10¹⁴ cm⁻³, preferably lower than 1×10¹² cm⁻³, more preferably lower than 1×10¹¹ cm⁻³ which is lower than or equal to the measurement limit is desirable. The carriers in the oxide semiconductor are reduced or preferably eliminated, in other words, the oxide semiconductor is made to be an i-type (intrinsic) semiconductor, so that the oxide semiconductor functions as a path in a transistor through which carriers pass. As a result, OFF-state current can be extremely small when the transistor is off. The above is the technical idea of this embodiment.

In addition, the oxide semiconductor functions as a path of carriers, and the oxide semiconductor itself is an i-type (intrinsic) semiconductor which is highly-purified so as to include no carriers or extremely few carriers, and thus carriers are supplied by a source electrode and a drain electrode.

The transistor having the structure described in this embodiment can less occupy a substrate surface than a horizontal transistor in which a channel is formed substantially in parallel with a substrate as described in Embodiment 4. As a result, it is possible to miniaturize the transistor.

As described above, the oxide semiconductor film is highly purified so that impurities other than a main component of the oxide semiconductor film, typically hydrogen, water, hydroxyl group, or hydride, may be contained as little as possible, whereby good operation of the transistor can be obtained. In particular, withstand voltage can be higher, a short channel effect can be reduced, and a high on-off ratio can be realized.

This embodiment can be implemented by being combined as appropriate with any of the above-described embodiments.

Embodiment 7

In this embodiment, a method for forming an oxide semiconductor film, which is different from that in Embodiment 4, is described with reference to FIGS. 15A to 15E.

First, a gate electrode 701 and a gate electrode 702 are formed over an insulating surface, and then a gate insulating film 703 is formed over the gate electrode 701 and the gate electrode 702 (see FIG. 15A). Since materials, structures, and thicknesses of the gate electrode 701, the gate electrode 702, and the gate insulating film 703 are already described in Embodiment 4, the detailed description thereof is omitted in this embodiment.

Next, as illustrated in FIG. 15A, a first oxide semiconductor film 730 with a thickness of greater than or equal to 2 nm and less than or equal to 15 nm is formed over the gate insulating film 703. The first oxide semiconductor film 730 can be formed by a sputtering method in a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere including a rare gas (for example, argon) and oxygen.

Note that before the first oxide semiconductor film 730 is formed by a sputtering method, dust on a surface of the gate insulating film 703 is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering means a method in which a surface of a substrate is modified by application of voltage to the substrate side, not to a target side, in an argon atmosphere with the use of an RF power source and by generating plasma in the vicinity of the substrate. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used.

For the first oxide semiconductor film 730, the above-described oxide semiconductor can be used.

In this embodiment, as the first oxide semiconductor film 730, an In—Ga—Zn—O-based non-single-crystal film with a thickness of 5 nm, which is obtained by a sputtering method using a metal oxide target including indium (In), gallium (Ga), and zinc (Zn), is used. As the target, for example, a metal oxide target having a composition ratio of metal atoms, In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. In this embodiment, a metal oxide target which tends to be crystallized is preferably used in order to intentionally perform crystallization by heat treatment performed later. The fill rate of the metal oxide target containing In, Ga, and Zn is higher than or equal to 90% and lower than or equal to 100%, and preferably higher than or equal to 95% and lower than or equal to 99.9%. When a metal oxide target having a high fill rate is used, the impurity concentration in an oxide semiconductor film to be formed can be reduced, so that a transistor with excellent electric characteristics or high reliability can be obtained.

The substrate is held in a treatment chamber kept under reduced pressure, a sputtering gas from which hydrogen and moisture are removed is introduced into the treatment chamber from which remaining moisture is being removed, and the first oxide semiconductor film 730 is formed over the insulating surface with the use of a metal oxide as a target. In film formation, the substrate temperature may be higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Film formation is performed while the substrate is heated, whereby the impurity concentration in the oxide semiconductor film formed can be reduced. In addition, damage by sputtering can be reduced. In order to remove moisture remaining in the treatment chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo pump provided with a cold trap. In the treatment chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), (more preferably, also a compound containing a carbon atom), and the like are removed, whereby the impurity concentration in the oxide semiconductor film formed in the treatment chamber can be reduced.

An example of the deposition condition is as follows: the distance between the substrate and the target is 170 mm, the pressure is 0.4 Pa, the direct current (DC) power source is 0.5 kW, and the atmosphere is an oxygen atmosphere (the flow rate of oxygen is 100%). Note that a pulse direct current (DC) power source is preferable because dust called particles generated in film formation can be reduced and the film thickness can be uniform. The thickness of the oxide semiconductor film is preferably greater than or equal to 5 nm and less than or equal to 30 nm. Since appropriate thickness depends on an oxide semiconductor material used, the thickness can be determined as appropriate depending on the material.

Note that in order that hydrogen, hydroxyl group, and moisture are contained as little as possible in the first oxide semiconductor film 730, it is preferable that the substrate over which layers up to and including the gate insulating film 703 are formed be preheated in a preheating chamber of the sputtering apparatus, so that impurities such as hydrogen or moisture absorbed onto the substrate is eliminated and evacuated, as pretreatment before film formation. The temperature for the preheating is higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an evacuation unit provided for the preheating chamber, a cryopump is preferable. Note that this preheating treatment can be omitted.

Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power supply, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case where an insulating film is formed, and a DC sputtering method is mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber.

Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge can be used.

Further, as a deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, or a bias sputtering method in which voltage is also applied to a substrate during deposition can be used.

The gate insulating film 703 and the first oxide semiconductor film 730 may be formed successively without exposure to air. Successive film formation without exposure to air makes it possible to obtain each interface between stacked layers, which is not contaminated by atmospheric components or impurity elements floating in air, such as water, hydrocarbon, or the like. Therefore, variation in characteristics of the transistor can be reduced.

Next, first heat treatment is performed and crystals are grown from a surface of the first oxide semiconductor film 730 as illustrated in FIG. 15B, so that a first oxide semiconductor film 731 in which at least a part thereof is crystallized or which becomes single crystals is obtained. The first heat treatment is performed at higher than or equal to 450° C. and lower than or equal to 850° C., preferably higher than or equal to 600° C. and lower than or equal to 700° C. Heating time is greater than or equal to 1 minute and less than or equal to 24 hours. The single crystal layer grows from the surface to the inside portion and contains plate-shaped crystals whose average thickness is greater than or equal to 2 nm and less than or equal to 10 nm. The crystal layer formed at the surface has the a-b plane and is c-axis oriented in a direction perpendicular to the surface. In this embodiment, an example in which the entire first oxide semiconductor film 731 is crystallized (the crystals are also referred to as co-growing (CG) crystals) by the first heat treatment is described.

Note that in the first heat treatment, it is preferable that water, hydrogen, and the like be not contained in nitrogen, oxygen, or a rare gas such as helium, neon, or argon. In addition, nitrogen, oxygen, or a rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus preferably has a purity of 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is, the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm). Further, the first heat treatment may be performed in a dry air atmosphere with an H₂O concentration of lower than or equal to 20 ppm.

In this embodiment, heat treatment in a dry air atmosphere at 700° C. for 1 hour is performed as the first heat treatment.

Note that a heat treatment apparatus is not limited to an electrical furnace, and may be provided with a device for heating an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon is used.

For example, the heat treatment can employ GRTA, in which the substrate is moved into an inert gas heated at a high temperature of 650° C. to 700° C., and heated for several minutes there, and then the substrate is moved out of the high-temperature inert gas. With GRTA, high-temperature heat treatment for a short period of time can be achieved.

Next, as illustrated in FIG. 15C, a second oxide semiconductor film 732 is formed over the first oxide semiconductor film 731, which includes plate-shaped single crystals. The thickness of the second oxide semiconductor film 732 is at least larger than that of the first oxide semiconductor film 731 and less than or equal to 10 μm. Note that an appropriate thickness of the second oxide semiconductor film 732 may be determined by a practitioner depending on a device manufactured. For example, in the case where a bottom-gate transistor is manufactured, the total thickness of the first oxide semiconductor film 731 and the second oxide semiconductor film 732 is greater than or equal to 10 nm and less than or equal to 200 nm. In addition, for example, in the case where a transistor is manufactured, the total thickness of the first oxide semiconductor film 731 and the second oxide semiconductor film 732 is greater than or equal to 10 nm and less than or equal to 50 nm. The second oxide semiconductor film 732 can be formed by a sputtering method in a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or an atmosphere including a rare gas (typically, argon) and oxygen.

The above oxide semiconductor can be used for the second oxide semiconductor film 732.

It is preferable that the first oxide semiconductor film 731 and the second oxide semiconductor film 732 be formed using materials containing the same components, or have the same crystal structure and close lattice constants (lattice mismatch is less than or equal to 1%). In the case where the materials containing the same components are used, crystal growth is further promoted from the single crystal layer of the first oxide semiconductor film 731 in the crystallization performed later. In addition, when the materials contain the same components, physical properties of an interface such as adhesion or electric characteristics are favorable. Note that the second oxide semiconductor film 732 is preferably formed using a material whose electric characteristics (e.g., mobility, threshold voltage, or band gap) is improved more than the first oxide semiconductor film 731 when crystallinity is increased.

Next, second heat treatment is performed, so that crystals are grown from the first oxide semiconductor film 731 toward the second oxide semiconductor film 732. The second heat treatment is performed at higher than or equal to 450° C. and lower than or equal to 850° C., preferably higher than or equal to 550° C. and lower than or equal to 650° C. Heating time is greater than or equal to 1 minute and less than or equal to 24 hours. By the second heat treatment, as illustrated in FIG. 15D, an oxide semiconductor film 733 including the crystallized first oxide semiconductor film 731 and a crystallized second oxide semiconductor film 735 can be obtained.

In the case where oxide semiconductor materials used for the first oxide semiconductor film 731 and the second oxide semiconductor film 735 contain the same components, the first oxide semiconductor film 731 and the second oxide semiconductor film 735 have the same crystal structure. The second oxide semiconductor film 735 is formed through axial growth or epitaxial growth from the first oxide semiconductor film 731, and thus, the c-axes of the second oxide semiconductor film 735 and the first oxide semiconductor film 731 are identical with each other. As a result, in the oxide semiconductor film 733, actually, the boundary between the first oxide semiconductor film 731 and the second oxide semiconductor film 735 is unclear.

In some cases, the oxide semiconductor film 733 includes polycrystals in a region overlapping with an uneven part of the gate insulating film, thereby including a crystal boundary. In addition, a region of the oxide semiconductor film 733 which serves as a channel formation region overlaps with at least a planar part of the gate insulating film, and therefore there is a case where the first oxide semiconductor film 731 and the second oxide semiconductor film 735 include c-axis-oriented single crystals. In the case where the first oxide semiconductor film 731 and the second oxide semiconductor film 735 are c-axis-oriented, the first oxide semiconductor film 731 and the second oxide semiconductor film 735 desirably have identical a-b planes, a-axes, or b-axes; however, the direction of the a-axes or b-axes may be different in some cases.

Note that also in the second heat treatment, it is preferable that water, hydrogen, and the like be not contained in nitrogen, oxygen, or a rare gas such as helium, neon, or argon. Alternatively, nitrogen, oxygen, or a rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus preferably has a purity of 6N or higher, more preferably 7N or higher (that is, the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm). Further, the second heat treatment may be performed in an ultra-dry air atmosphere with an H₂O concentration of 20 ppm or lower. In addition, at the time of increasing the temperature in the second heat treatment, an atmosphere of a furnace may be a nitrogen atmosphere and the atmosphere may be switched to an oxygen atmosphere at the time of performing cooling.

Note that the heat treatment apparatus for the second heat treatment is not limited to a particular apparatus, and the apparatus may be provided with a device for heating an object to be processed by heat radiation or heat conduction from a heating element such as a resistance heating element. For example, an electric furnace, or an RTA apparatus such as a GRTA apparatus or an LRTA apparatus can be used.

Next, the shape of the oxide semiconductor film 733 is processed by a photolithography method, so that an island-shaped oxide semiconductor film 734 and an island-shaped oxide semiconductor film 736 are formed to overlap with the gate electrode 701 and the gate electrode 702, respectively. Note that a resist mask for forming the island-shaped oxide semiconductor films may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

In accordance with the steps after the step for forming the source electrode and the drain electrode illustrated in FIG. 11A of Embodiment 4, a transistor which serves as a switching element of a memory cell can be manufactured.

This embodiment can be implemented by being combined as appropriate with any of the above-described embodiments.

Embodiment 8

In this embodiment, a structure of a CPU, one of semiconductor devices each of which is one embodiment of the present invention, will be described.

In FIG. 17, the structure of the CPU of this embodiment is illustrated. The CPU shown in FIG. 17 mainly includes an arithmetic logic unit (ALU) 901, an ALU controller 902, an instruction decoder 903, an interrupt controller 904, a timing controller 905, a register 906, a register controller 907, a bus interface (Bus I/F) 908, a rewritable ROM 909 and a ROM interface (ROM I/F) 920, over a substrate 900. Further, the ROM 909 and the ROM I/F 920 may be provided over another chip. Needless to say, the CPU shown in FIG. 17 is only an example in which the configuration is simplified, and an actual CPU may have various configurations depending on the uses.

An instruction which is input to the CPU through the Bus I/F 908 is input to the instruction decoder 903 and decoded therein, and then, input to the ALU controller 902, the interrupt controller 904, the register controller 907, and the timing controller 905.

The ALU controller 902, the interrupt controller 904, the register controller 907, and the timing controller 905 conduct various controls based on the decoded instruction. Specifically, the ALU controller 902 generates signals for controlling the drive of the ALU 901. While the CPU is executing a program, the interrupt controller 904 judges an interrupt request from an external input/output device or a peripheral circuit based on its priority or a mask state, and processes the request. The register controller 907 generates an address of the register 906, and reads/writes data from/to the register 906 in accordance with the state of the CPU.

The timing controller 905 generates signals for controlling a drive timing of the ALU 901, the ALU controller 902, the instruction decoder 903, the interrupt controller 904, and the register controller 907. For example, the timing controller 905 is provided with an internal clock generator for generating an internal clock signal CLK2 based on a reference clock signal CLK1, and supplies the clock signal CLK2 to the various above circuits.

In the CPU of this embodiment, a memory element having the structure described in any of the above embodiments is provided in the register 906. The register controller 907 selects the holding operation in the register 906 in accordance with an instruction from the ALU 901. That is, in the memory element included in the register 906, whether data is held in a phase-inversion element or a capacitor is selected. In the case where selection is made so that data is held in the phase-inversion element, a power supply voltage is applied to the memory element in the register 906. In the case where selection is made so that data is held in the capacitor, the data is written into the capacitor, and the application of the power supply voltage to the memory element in the register 906 can be stopped. The power supply can be stopped by providing a switching element between a memory element group and a node to which a power supply potential VDD or a power supply potential VSS is supplied, as illustrated in FIG. 13A or FIG. 13B.

In such a manner, even in the case where the operation of the CPU is temporally stopped and the application of the power supply voltage is stopped, data can be held and power consumption can be reduced. Specifically, for example, while a user of a personal computer does not input data to an input device such as a keyboard, the operation of the CPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example in this embodiment, the semiconductor device of the present invention is not limited to the CPU, but can be applied to an LSI such as a DSP, a custom LSI, or a field programmable gate array (FPGA).

This embodiment can be implemented by being combined as appropriate with any of the above-described embodiments.

Example 1

A semiconductor device which is one embodiment of the present invention is used, so that a highly reliable electronic device and an electronic device with low power consumption can be provided. In particular, in the case of a portable electronic device which has difficulty in continuously receiving electric power, a semiconductor device with low power consumption which is one embodiment of the present invention is added to the component of the device, whereby an advantage in increasing the continuous operation time can be obtained. Further, with the use of a transistor with low OFF-state current, redundant circuit design which is needed to cover a failure caused by high OFF-state current is unnecessary; therefore, the density of an integrated circuit used for the semiconductor device can be increased, and the semiconductor device can have a higher performance.

A semiconductor device which is one embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). In addition, as an electronic device which can include the semiconductor device which is one embodiment of the present invention, mobile phones, game machines including portable game machines, portable information terminals, e-book readers, video cameras, digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), vending machines, and the like can be given. Specific examples of these electronic devices are shown in FIGS. 18A to 18F.

FIG. 18A illustrates an e-book reader including a housing 7001, a display portion 7002, and the like. A semiconductor device which is one embodiment of the present invention can be used for an integrated circuit which controls driving of the e-book reader. With the use of the semiconductor device which is one embodiment of the present invention for the integrated circuit which controls driving of the e-book reader, a highly reliable e-book reader and an e-book reader with low power consumption can be provided. When a flexible substrate is used, a semiconductor device and a semiconductor display device can have flexibility, whereby a user-friendly e-book reader which is flexible and lightweight can be provided.

FIG. 18B illustrates a display device including a housing 7011, a display portion 7012, a supporting base 7013, and the like. A semiconductor device which is one embodiment of the present invention can be used for an integrated circuit which controls driving of the display device. With the use of the semiconductor device which is one embodiment of the present invention for the integrated circuit which controls driving of the display device, a highly reliable display device and a display device with low power consumption can be provided. Note that a display device includes all display devices for displaying information, such as display devices for personal computers, for receiving television broadcast, and for displaying advertisement, in its category.

FIG. 18C illustrates a display device including a housing 7021, a display portion 7022, and the like. A semiconductor device which is one embodiment of the present invention can be used for an integrated circuit which controls driving of the display device. With the use of the semiconductor device which is one embodiment of the present invention for the integrated circuit which controls driving of the display device, a highly reliable display device and a display device with low power consumption can be provided. When a flexible substrate is used, a semiconductor device and a semiconductor display device can have flexibility, whereby a user-friendly display device which is flexible and lightweight can be provided. Accordingly, as illustrated in FIG. 18C, a display device can be used while being fixed to fabric or the like, and an application range of the semiconductor display device is dramatically widened.

FIG. 18D illustrates a portable game machine including a housing 7031, a housing 7032, a display portion 7033, a display portion 7034, a microphone 7035, speakers 7036, operation keys 7037, a stylus 7038, and the like. A semiconductor device which is one embodiment of the present invention can be used for an integrated circuit which controls driving of the portable game machine. With the use of the semiconductor device which is one embodiment of the present invention for the integrated circuit which controls driving of the portable game machine, a highly reliable portable game machine and a portable game machine with low power consumption can be provided. Although the portable game machine illustrated in FIG. 18D has the two display portions 7033 and 7034, the number of display portions included in the portable game machines is not limited thereto.

FIG. 18E illustrates a mobile phone including a housing 7041, a display portion 7042, an audio-input portion 7043, an audio-output portion 7044, operation keys 7045, a light-receiving portion 7046, and the like. Light received in the light-receiving portion 7046 is converted into electrical signals, whereby external images can be loaded. A semiconductor device which is one embodiment of the present invention can be used for an integrated circuit which controls driving of the mobile phone. With the use of the semiconductor device which is one embodiment of the present invention for the integrated circuit which controls driving of the mobile phone, a highly reliable mobile phone and a mobile phone with low power consumption can be provided.

FIG. 18F illustrates a portable information terminal including a housing 7051, a display portion 7052, operation keys 7053, and the like. In the portable information terminal illustrated in FIG. 18F, a modem may be incorporated in the housing 7051. A semiconductor device which is one embodiment of the present invention can be used for an integrated circuit which controls driving of the portable information terminal. With the use of the semiconductor device which is one embodiment of the present invention for the integrated circuit which controls driving of the potable information terminal, a highly reliable portable information terminal and a portable information terminal with low power consumption can be provided.

Example 1 can be implemented by being combined as appropriate with any of the above-described embodiments.

This application is based on Japanese Patent Application serial no. 2009-293982 filed with Japan Patent Office on Dec. 25, 2009, the entire contents of which are hereby incorporated by reference. 

1. (canceled)
 2. A memory device comprising: a first transistor; and a memory element group comprising a plurality of memory elements, each of the plurality of memory elements comprising: a first logic element comprising an input terminal and an output terminal; and a second logic element comprising an input terminal and an output terminal, wherein the input terminal of the first logic element is electrically connected to the output terminal of the second logic element, wherein the input terminal of the second logic element is electrically connected to the output terminal of the first logic element, wherein each of the plurality of memory elements is configured to be supplied with a high-level power supply potential and a low-level power supply potential, wherein one of the high-level power supply potential and the low-level power supply potential is supplied to each of the plurality of memory elements through a source of the first transistor and a drain of the first transistor, wherein each of the first logic element and the second logic element is an inverter or a clocked inverter, and wherein the first transistor comprises a channel formation region comprising an oxide semiconductor.
 3. The memory device according to claim 2, wherein each of the first logic element and the second logic element is configured to be supplied with the high-level power supply potential and the low-level power supply potential.
 4. The memory device according to claim 2, wherein each of the plurality of memory elements comprises a second transistor comprising a first terminal, a second terminal, and a gate, wherein the input terminal of the first logic element is electrically connected to the output terminal of the second logic element through the first terminal of the second transistor and the second terminal of the second transistor, and wherein the second transistor comprises a channel formation region comprising an oxide semiconductor.
 5. The memory device according to claim 2, wherein each of the plurality of memory elements comprises: a second transistor comprising a first terminal, a second terminal, and a gate; and a first capacitor comprising a first terminal and a second terminal, wherein the first terminal of the second transistor is electrically connected to the input terminal of the first logic element and the output terminal of the second logic element, wherein the second terminal of the second transistor is electrically connected to the first terminal of the first capacitor, and wherein the second transistor comprises a channel formation region comprising an oxide semiconductor.
 6. The memory device according to claim 5, wherein each of the plurality of memory elements comprises: a third transistor comprising a first terminal, a second terminal, and a gate; and a second capacitor comprising a first terminal and a second terminal, wherein the first terminal of the third transistor is electrically connected to the output terminal of the first logic element and the input terminal of the second logic element, wherein the second terminal of the third transistor is electrically connected to the first terminal of the second capacitor, and wherein the third transistor comprises a channel formation region comprising an oxide semiconductor.
 7. The memory device according to claim 2, wherein each of the plurality of memory elements comprises: a second transistor comprising a first terminal, a second terminal, and a gate; and a capacitor comprising a first terminal and a second terminal, wherein the first terminal of the second transistor is electrically connected to the output terminal of the second logic element, wherein the second terminal of the second transistor is electrically connected to the first terminal of the capacitor and the input terminal of the first logic element, and wherein the second transistor comprise a channel formation region comprising an oxide semiconductor.
 8. The memory device according to claim 2, wherein the first transistor comprises: a first electrode over an insulating surface; an oxide semiconductor film comprising the channel formation region over the first electrode; a second electrode over the oxide semiconductor film; a gate insulating film covering the first electrode, the oxide semiconductor film, and the second electrode; and a gate electrode over the gate insulating film, wherein one of the first electrode and the second electrode is the source, and wherein the other of the first electrode and the second electrode is the drain.
 9. The memory device according to claim 2, wherein the plurality of memory elements are connected in series.
 10. The memory device according to claim 2, wherein each of the plurality of memory elements is configured to be supplied with a potential of a signal.
 11. The memory device according to claim 2, wherein the oxide semiconductor comprises indium, gallium, and zinc.
 12. A CPU comprising: a register comprising a memory device comprising: a first transistor; and a memory element group comprising a plurality of memory elements, each of the plurality of memory elements comprising: a first logic element comprising an input terminal and an output terminal; and a second logic element comprising an input terminal and an output terminal, wherein the input terminal of the first logic element is electrically connected to the output terminal of the second logic element, wherein the input terminal of the second logic element is electrically connected to the output terminal of the first logic element, wherein each of the plurality of memory elements is configured to be supplied with a high-level power supply potential and a low-level power supply potential, wherein one of the high-level power supply potential and the low-level power supply potential is supplied to each of the plurality of memory elements through a source and a drain of the first transistor, wherein each of the first logic element and the second logic element is an inverter or a clocked inverter, and wherein the first transistor comprises a channel formation region comprising an oxide semiconductor.
 13. The CPU according to claim 12, wherein each of the first logic element and the second logic element is configured to be supplied with the high-level power supply potential and the low-level power supply potential.
 14. The CPU according to claim 12, wherein each of the plurality of memory elements comprises a second transistor comprising a first terminal, a second terminal, and a gate, wherein the input terminal of the first logic element is electrically connected to the output terminal of the second logic element through the first terminal of the second transistor and the second terminal of the second transistor, and wherein the second transistor comprises a channel formation region comprising an oxide semiconductor.
 15. The CPU according to claim 12, wherein each of the plurality of memory elements comprises: a second transistor comprising a first terminal, a second terminal, and a gate; and a first capacitor comprising a first terminal and a second terminal, wherein the first terminal of the second transistor is electrically connected to the input terminal of the first logic element and the output terminal of the second logic element, wherein the second terminal of the second transistor is electrically connected to the first terminal of the first capacitor, and wherein the second transistor comprises a channel formation region comprising an oxide semiconductor.
 16. The CPU according to claim 15, wherein each of the plurality of memory elements comprises: a third transistor comprising a first terminal, a second terminal, and a gate; and a second capacitor comprising a first terminal and a second terminal, wherein the first terminal of the third transistor is electrically connected to the output terminal of the first logic element and the input terminal of the second logic element, wherein the second terminal of the third transistor is electrically connected to the first terminal of the second capacitor, and wherein the third transistor comprises a channel formation region comprising an oxide semiconductor.
 17. The CPU according to claim 12, wherein each of the plurality of memory elements comprises: a second transistor comprising a first terminal, a second terminal, and a gate; and a capacitor comprising a first terminal and a second terminal, wherein the first terminal of the second transistor is electrically connected to the output terminal of the second logic element, wherein the second terminal of the second transistor is electrically connected to the first terminal of the capacitor and the input terminal of the first logic element, and wherein the second transistor comprise a channel formation region comprising an oxide semiconductor.
 18. The CPU according to claim 12, wherein the first transistor comprises: a first electrode over an insulating surface; an oxide semiconductor film comprising the channel formation region over the first electrode; a second electrode over the oxide semiconductor film; a gate insulating film covering the first electrode, the oxide semiconductor film, and the second electrode; and a gate electrode over the gate insulating film, wherein one of the first electrode and the second electrode is the source, and wherein the other of the first electrode and the second electrode is the drain.
 19. The CPU according to claim 12, wherein the plurality of memory elements are connected in series.
 20. The CPU according to claim 12, wherein each of the plurality of memory elements is configured to be supplied with a potential of a signal.
 21. The CPU according to claim 12, wherein the oxide semiconductor comprises indium, gallium, and zinc. 